What is the difference between "INNER JOIN" and "OUTER JOIN"?

Sure! I'd be happy to explain the difference between INNER JOIN and OUTER JOIN queries in SQL, as well as describe the role of LEFT OUTER JOIN, RIGHT OUTER JOIN, and FULL OUTER JOIN.

First, let me begin with a brief explanation of what JOIN operations do in general. In SQL, a join operation combines rows from two or more tables based on a related column between them, producing a result set that includes data from both tables.

Now, let's dive into the specifics of INNER JOIN and OUTER JOIN.

INNER JOIN: The simplest type of join operation, which returns only rows where there is a match in both tables based on their join keys. No unmatched rows are included from either table in the result set.

In other words, an INNER JOIN query searches for matching records in both tables and combines them into a single result, while ignoring any discrepancies that may exist. This is ideal when you need to find specific data from multiple tables, knowing that it exists in both table with a common key.

OUTER JOIN, on the other hand: Expands beyond just returning records with matches in both tables, and also includes records with no matching rows from one or even both of the joined tables. The two types of OUTER JOIN are LEFT OUTER JOIN and RIGHT OUTER JOIN, each returning different results:

• LEFT OUTER JOIN (also known as Left Join): This join operation returns all records from the left table (i.e., the table specified first in the query), and only those records from the right table that have a match based on their join key. If there is no matching record, then NULL values fill any columns of data from the right table in the result set. This is useful when you want to get all data from one table, but only the related data from another table if it exists.

• RIGHT OUTER JOIN (also known as Right Join): Reverse of a Left Outer Join. It returns all records from the right table, along with matching records from the left table based on their join keys. If there is no match, then NULL values are returned for the columns of data from the left table in the result set. This is helpful when you need to get all data from a given table but include all related records (if any exist) from another table as well.

Finally, there's a FULL OUTER JOIN, also known as Full Outer Join, which does precisely what both LEFT and RIGHT OUTER JOIN do together: It returns all records that have matches in either the left or right table based on their join keys. If there is no matching record from either table, then NULL values fill any columns of data not found in that respective table in the result set.

In summary:

• INNER JOIN: Returns only rows where there are matches in both tables based on their join keys. No unmatched rows are included in the result set.
• LEFT OUTER JOIN/Left Join: Returns all records from the left table and matching records from the right table (based on their join keys). If no match is found, NULL values fill any columns of data from the right table.
• RIGHT OUTER JOIN/Right Join: Returns all records from the right table along with matching records from the left table. If no match is found in the left table, NULL values will fill any columns of data from that table.
• FULL OUTER JOIN/Full Outer Join: A combination of both LEFT OUTER JOIN and RIGHT OUTER JOIN, returns all records from both tables, based on their join keys or NULL values if there is no match.

Assuming you're joining on columns with no duplicates, which is a very common case:

• An inner join of A and B gives the result of A intersect B, i.e. the inner part of a Venn diagram intersection.- An outer join of A and B gives the results of A union B, i.e. the outer parts of a Venn diagram union.

Suppose you have two tables, with a single column each, and data as follows:

A    B
-    -
1    3
2    4
3    5
4    6

Note that (1,2) are unique to A, (3,4) are common, and (5,6) are unique to B.

An inner join using either of the equivalent queries gives the intersection of the two tables, i.e. the two rows they have in common.

select * from a INNER JOIN b on a.a = b.b;
select a.*, b.*  from a,b where a.a = b.b;

a | b
--+--
3 | 3
4 | 4

A left outer join will give all rows in A, plus any common rows in B.

select * from a LEFT OUTER JOIN b on a.a = b.b;
select a.*, b.*  from a,b where a.a = b.b(+);

a |  b
--+-----
1 | null
2 | null
3 |    3
4 |    4

A right outer join will give all rows in B, plus any common rows in A.

select * from a RIGHT OUTER JOIN b on a.a = b.b;
select a.*, b.*  from a,b where a.a(+) = b.b;

a    |  b
-----+----
3    |  3
4    |  4
null |  5
null |  6

A full outer join will give you the union of A and B, i.e. all the rows in A and all the rows in B. If something in A doesn't have a corresponding datum in B, then the B portion is null, and vice versa.

select * from a FULL OUTER JOIN b on a.a = b.b;

 a   |  b
-----+-----
   1 | null
   2 | null
   3 |    3
   4 |    4
null |    6
null |    5

Certainly! Let's dive into the differences between INNER JOIN and OUTER JOIN, and how the different types of OUTER JOIN fit into the picture.

1. INNER JOIN:

   • An INNER JOIN returns a result set that contains only the rows that have matching values in both tables being joined.
   • It combines rows from two or more tables based on a related column between them and returns a result set that contains only the matched rows.
   • The INNER JOIN operation can be written using the INNER JOIN keyword or simply JOIN keyword.
   • Example:

     SELECT customers.name, orders.order_date
     FROM customers
     INNER JOIN orders
     ON customers.customer_id = orders.customer_id;
     

2. OUTER JOIN:

   • An OUTER JOIN returns a result set that contains all rows from one table and the matched rows from the other table.
   • There are three types of OUTER JOIN:
     • LEFT OUTER JOIN: Returns all rows from the left table and the matching rows from the right table.
     • RIGHT OUTER JOIN: Returns all rows from the right table and the matching rows from the left table.
     • FULL OUTER JOIN: Returns all rows from both tables, regardless of whether there is a match or not.
   • Example:

     -- LEFT OUTER JOIN
     SELECT customers.name, orders.order_date
     FROM customers
     LEFT OUTER JOIN orders
     ON customers.customer_id = orders.customer_id;
     
     -- RIGHT OUTER JOIN
     SELECT customers.name, orders.order_date
     FROM customers
     RIGHT OUTER JOIN orders
     ON customers.customer_id = orders.customer_id;
     
     -- FULL OUTER JOIN
     SELECT customers.name, orders.order_date
     FROM customers
     FULL OUTER JOIN orders
     ON customers.customer_id = orders.customer_id;
     

The key differences between INNER JOIN and OUTER JOIN are:

1. Result Set: INNER JOIN returns only the rows that have matching values in both tables, while OUTER JOIN returns all rows from one table and the matching rows from the other table.

2. Handling Unmatched Rows: In an INNER JOIN, the unmatched rows from either table are not included in the result set. In an OUTER JOIN, the unmatched rows from the specified table(s) are included in the result set, with null values for the columns from the other table(s).

3. Types of Outer Joins: As mentioned, there are three types of OUTER JOIN: LEFT OUTER JOIN, RIGHT OUTER JOIN, and FULL OUTER JOIN, each with its own behavior in terms of which table's rows are included in the result set.

The choice between INNER JOIN and OUTER JOIN (and the specific type of OUTER JOIN) depends on the requirements of your query and the data you're working with. INNER JOIN is useful when you only want to see the matching rows, while OUTER JOIN is useful when you want to include all rows from one or both tables, even if there are no matching rows.

The main difference between an INNER JOIN and an OUTER JOIN is that an INNER JOIN only returns rows where there is a match between the joined tables, while an OUTER JOIN can return rows even if there is no match in the other table.

Let's break it down step by step:

1. INNER JOIN:

   • An INNER JOIN returns only the rows where there is a match between the joined tables based on the specified join condition.
   • If a row in one table doesn't have a matching row in the other table, it will not be included in the result set.
   • The syntax for an INNER JOIN is:

     SELECT columns
     FROM table1
     INNER JOIN table2 ON table1.column = table2.column;
     

2. OUTER JOIN:

   • An OUTER JOIN returns all the rows from one table and the matching rows from the other table. If there is no match, NULL values are returned for the columns of the non-matching table.
   • There are three types of OUTER JOINs:
     • LEFT OUTER JOIN (or simply LEFT JOIN):
       • Returns all the rows from the left table and the matching rows from the right table.
       • If there is no match in the right table, NULL values are returned for the right table's columns.
       • The syntax for a LEFT OUTER JOIN is:

         SELECT columns
         FROM table1
         LEFT OUTER JOIN table2 ON table1.column = table2.column;
         

     • RIGHT OUTER JOIN (or simply RIGHT JOIN):
       • Returns all the rows from the right table and the matching rows from the left table.
       • If there is no match in the left table, NULL values are returned for the left table's columns.
       • The syntax for a RIGHT OUTER JOIN is:

         SELECT columns
         FROM table1
         RIGHT OUTER JOIN table2 ON table1.column = table2.column;
         

     • FULL OUTER JOIN (or simply FULL JOIN):
       • Returns all the rows from both tables, regardless of whether there is a match or not.
       • If there is no match in either table, NULL values are returned for the non-matching columns.
       • The syntax for a FULL OUTER JOIN is:

         SELECT columns
         FROM table1
         FULL OUTER JOIN table2 ON table1.column = table2.column;
         

In summary:

• INNER JOIN returns only the matching rows between the joined tables.
• LEFT OUTER JOIN returns all the rows from the left table and the matching rows from the right table, with NULL values for non-matching columns in the right table.
• RIGHT OUTER JOIN returns all the rows from the right table and the matching rows from the left table, with NULL values for non-matching columns in the left table.
• FULL OUTER JOIN returns all the rows from both tables, with NULL values for non-matching columns in either table.

The choice of join type depends on your specific requirements and the desired result set based on the relationship between the tables being joined.

The difference between INNER JOIN and OUTER JOIN lies in how they handle rows from the joined tables when there is no matching row in the other table.

INNER JOIN: An INNER JOIN returns only the rows from both tables where the join condition is met. In other words, it returns the intersection of the two tables based on the join condition. If a row from one table does not have a matching row in the other table, it is excluded from the result set.

Example:

SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
INNER JOIN Orders ON Customers.CustomerID = Orders.CustomerID;

This query will return only the customer names and order IDs for customers who have placed orders.

OUTER JOIN: OUTER JOIN includes rows from one or both tables, even if there are no matching rows in the other table. There are three types of OUTER JOIN:

1. LEFT OUTER JOIN (or LEFT JOIN): Returns all rows from the left table (the one before the JOIN keyword), and the matching rows from the right table. If there are no matching rows in the right table, the result will contain NULL values for those columns.

Example:

SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
LEFT JOIN Orders ON Customers.CustomerID = Orders.CustomerID;

This query will return all customer names, and their order IDs if they have placed orders. For customers who have not placed any orders, the OrderID column will be NULL.

2. RIGHT OUTER JOIN (or RIGHT JOIN): Returns all rows from the right table (the one after the JOIN keyword), and the matching rows from the left table. If there are no matching rows in the left table, the result will contain NULL values for those columns.

Example:

SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
RIGHT JOIN Orders ON Customers.CustomerID = Orders.CustomerID;

This query will return all order IDs, and the customer names for those orders if they exist. If an order does not have a matching customer, the CustomerName column will be NULL.

3. FULL OUTER JOIN: Returns all rows from both tables, combining the results of LEFT JOIN and RIGHT JOIN. If there are no matching rows in one of the tables, the result will contain NULL values for those columns.

Example:

SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
FULL OUTER JOIN Orders ON Customers.CustomerID = Orders.CustomerID;

This query will return all customer names and order IDs. If a customer has no orders, the OrderID column will be NULL. If an order has no matching customer, the CustomerName column will be NULL.

In summary, INNER JOIN returns only the intersecting rows, while OUTER JOIN (including LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN) includes rows from one or both tables, even if there are no matching rows in the other table.

In SQL, INNER JOIN and OUTER JOIN are used to combine rows from two or more tables based on a related column between them. The main difference between them lies in how they handle rows that do not have a match in the joined table.

INNER JOIN: An INNER JOIN returns only the matching rows from both tables. If there is no match in one of the tables, those rows will not be included in the result set.

For example, let's say we have two tables, Orders and Customers. The Orders table has columns OrderID and CustomerID, and the Customers table has columns CustomerID and CustomerName. An INNER JOIN query would look like this:

SELECT Orders.OrderID, Customers.CustomerName
FROM Orders
INNER JOIN Customers
ON Orders.CustomerID = Customers.CustomerID;

This query returns only the orders that have a matching customer in the Customers table.

OUTER JOIN: An OUTER JOIN returns all rows from at least one of the tables, even if there is no match in the other table. There are three types of OUTER JOINs: LEFT OUTER JOIN, RIGHT OUTER JOIN, and FULL OUTER JOIN.

1. LEFT OUTER JOIN: A LEFT OUTER JOIN returns all rows from the left table (the one appearing before the JOIN keyword) and matching rows from the right table. If there is no match in the right table, NULL values will be displayed for the right table's columns.

Example:

SELECT Orders.OrderID, Customers.CustomerName
FROM Orders
LEFT OUTER JOIN Customers
ON Orders.CustomerID = Customers.CustomerID;

This query returns all orders, including those with no matching customer in the Customers table.

2. RIGHT OUTER JOIN: A RIGHT OUTER JOIN is similar to a LEFT OUTER JOIN, but with the tables reversed. It returns all rows from the right table and matching rows from the left table. If there is no match in the left table, NULL values will be displayed for the left table's columns.

Example:

SELECT Orders.OrderID, Customers.CustomerName
FROM Orders
RIGHT OUTER JOIN Customers
ON Orders.CustomerID = Customers.CustomerID;

This query returns all customers, including those with no matching order in the Orders table.

3. FULL OUTER JOIN: A FULL OUTER JOIN returns all rows from both tables, regardless of whether there is a match. If there is no match in one of the tables, NULL values will be displayed for the missing table's columns.

Example:

SELECT Orders.OrderID, Customers.CustomerName
FROM Orders
FULL OUTER JOIN Customers
ON Orders.CustomerID = Customers.CustomerID;

This query returns all orders and all customers, with NULL values for orders with no matching customer and customers with no matching order.

To summarize, INNER JOIN returns only matched rows, while OUTER JOINs return matched rows and unmatched rows from one or both tables. The type of OUTER JOIN used (LEFT, RIGHT, or FULL) determines which table's unmatched rows are returned.

The INNER JOIN statement returns only the rows in the two tables that share the same values in the matching columns. An OUTER JOIN returns all the rows from the two tables, and combines them with the matching columns. Another type of outer join is a LEFT OUTER JOIN. A left outer join returns all rows from the left table, and those rows in the right table that share values on the specified column or columns. If there are no matches between the tables, NULLs will be returned for those columns. A RIGHT OUTER JOIN does the same thing as a LEFT OUTER JOIN, but starts with the right table and combines it with all the rows in the left table.

Another type of outer join is a FULL OUTER JOIN. This returns both the rows from the left and right tables, and all values that they do not share will be returned as nulls.

Difference between INNER JOIN and OUTER JOIN

INNER JOIN

• Returns only rows where the join condition is satisfied for both tables.
• Filters out rows from both tables that do not have matching values in the join column(s).

OUTER JOIN

• Returns rows from one or both tables, even if there are no matching values in the join column(s).
• There are three types of OUTER JOINs:

Types of OUTER JOINs

LEFT OUTER JOIN

• Returns all rows from the left table and only the matching rows from the right table.
• If there is no matching row in the right table, the right table columns will be filled with NULL values.

RIGHT OUTER JOIN

• Returns all rows from the right table and only the matching rows from the left table.
• If there is no matching row in the left table, the left table columns will be filled with NULL values.

FULL OUTER JOIN

• Returns all rows from both tables, regardless of whether there are matching values in the join column(s).
• If there is no matching row in one of the tables, the columns from that table will be filled with NULL values.

Example

Consider the following tables:

Table1:
| id | name |
|---|---|
| 1  | John  |
| 2  | Mary  |
| 3  | Bob   |

Table2:
| id | address |
|---|---|
| 1  | 123 Main St  |
| 2  | 456 Elm St   |

INNER JOIN

SELECT *
FROM Table1
INNER JOIN Table2 ON Table1.id = Table2.id;

Result:

| id | name | address |
|---|---|---|
| 1  | John  | 123 Main St  |
| 2  | Mary  | 456 Elm St   |

Rows 3 and 4 from Table1 are not included because they do not have matching rows in Table2.

LEFT OUTER JOIN

SELECT *
FROM Table1
LEFT OUTER JOIN Table2 ON Table1.id = Table2.id;

Result:

| id | name | address |
|---|---|---|
| 1  | John  | 123 Main St  |
| 2  | Mary  | 456 Elm St   |
| 3  | Bob   | NULL        |

Row 3 from Table1 is included even though it does not have a matching row in Table2. The address column is filled with NULL.

RIGHT OUTER JOIN

SELECT *
FROM Table1
RIGHT OUTER JOIN Table2 ON Table1.id = Table2.id;

Result:

| id | name | address |
|---|---|---|
| 1  | John  | 123 Main St  |
| 2  | Mary  | 456 Elm St   |
| 4  | NULL  | 456 Elm St   |

Row 4 from Table2 is included even though it does not have a matching row in Table1. The name column is filled with NULL.

FULL OUTER JOIN

SELECT *
FROM Table1
FULL OUTER JOIN Table2 ON Table1.id = Table2.id;

Result:

| id | name | address |
|---|---|---|
| 1  | John  | 123 Main St  |
| 2  | Mary  | 456 Elm St   |
| 3  | Bob   | NULL        |
| 4  | NULL  | 456 Elm St   |

Both rows from Table1 and Table2 are included, even though they do not have matching values in the join column. NULL values are filled in for the columns that do not have matching values.

Sure, here's the difference between INNER JOIN and OUTER JOIN:

INNER JOIN:

• Joins two tables by comparing columns with the same name.
• Returns rows where columns are present in both tables.

Outer JOIN:

• Joins two tables, but includes rows from both tables, even if there are no matching columns.

• There are three types of outer join:

  • LEFT OUTER JOIN: Includes rows from the left table, even if there are no matching columns in the right table.
  • RIGHT OUTER JOIN: Includes rows from the right table, even if there are no matching columns in the left table.
  • FULL OUTER JOIN: Includes all rows from both tables, regardless of whether they have matching columns.

Example:

INNER JOIN:

SELECT *
FROM employees
INNER JOIN departments
ON employees.department_id = departments.id

This will return employees and departments where the department ID is the same.

LEFT OUTER JOIN:

SELECT *
FROM employees
LEFT OUTER JOIN departments
ON employees.department_id = departments.id

This will return all employees, even if they don't have a department. Departments without employees will have NULL values for the department name.

Right Outer Join:

SELECT *
FROM employees
RIGHT OUTER JOIN departments
ON employees.department_id = departments.id

This will return all departments, even if there are no employees in that department. Employees without departments will have NULL values for the employee name.

Full Outer Join:

SELECT *
FROM employees
FULL OUTER JOIN departments
ON employees.department_id = departments.id

This will return all rows from both tables, regardless of whether they have matching columns. Rows that don't have matching columns will have NULL values.

"INNER JOIN", "LEFT JOIN", and "RIGHT JOIN" are types of joins used in SQL to combine rows from two or more tables. However, they differ in the way they handle missing matches between the tables being joined:

1. INNER JOIN returns records that have matching values in both tables. If there is no match, it won’t return either one (this can also be called an “inner” join because it focuses on matching rows only). It's denoted by a simple keyword 'JOIN'. Example:

SELECT Orders.OrderID, Customers.CustomerName 
FROM Orders 
INNER JOIN Customers ON Orders.CustomerId = Customers.CustomerId;

2. LEFT (OUTER) JOIN returns all the rows from the left table and matches with the right table. If there is no match, it still includes the row from the left table but in columns of the matched row are populated by NULL values. An OUTER keyword 'JOIN' before LEFT means it’s an outer join. Example:

SELECT Orders.OrderID, Customers.CustomerName 
FROM Orders 
LEFT JOIN Customers ON Orders.CustomerId = Customers.CustomerId;

3. RIGHT (OUTER) JOIN is similar to the LEFT join, except it returns all rows from the right table and matches with the left table. If there's no match in the left table for a row of the right one, then that will be included in the result set but columns on matching row populated by NULL values. An OUTER keyword 'JOIN' before RIGHT means it’s an outer join. Example:

SELECT Orders.OrderID, Customers.CustomerName 
FROM Orders 
RIGHT JOIN Customers ON Orders.CustomerId = Customers.CustomerId;

4. FULL (OUTER) JOIN returns all records from both left and right tables. If there's no match, NULL is used to fill the missing values on either side of the join result set. An OUTER keyword 'JOIN' before FULL means it’s an outer join. Example:

SELECT Orders.OrderID, Customers.CustomerName 
FROM Orders 
FULL JOIN Customers ON Orders.CustomerId = Customers.CustomerId;

So to sum up, the difference is primarily about how NULL values are handled and what rows get included in the result set - all four types of join behave differently when handling missing matches between the tables being joined.

INNER JOIN

• An INNER JOIN combines rows from two tables where the matching columns have the same values.
• It only returns rows where the join condition is met in both tables.
• It can be used to find exact matches between rows.

Example:

SELECT * FROM table1 INNER JOIN table2 ON table1.id = table2.id

OUTER JOIN

• An OUTER JOIN combines rows from two tables, even if there is no matching value.
• It returns all rows from the left table, even if there is no match in the right table.
• It can be used to find all possible matches between rows in the two tables.

Example:

SELECT * FROM table1 OUTER JOIN table2 ON table1.id = table2.id

LEFT OUTER JOIN

• A LEFT OUTER JOIN combines rows from two tables, including all rows from the left table and matching rows from the right table.
• It returns all rows from the left table, even if there is no matching value in the right table.
• It can be used to find all possible matches between rows in the two tables.

Example:

SELECT * FROM table1 LEFT OUTER JOIN table2 ON table1.id = table2.id

RIGHT OUTER JOIN

• A RIGHT OUTER JOIN combines rows from two tables, including all rows from the right table and matching rows from the left table.
• It returns all rows from the right table, even if there is no matching value in the left table.
• It can be used to find all possible matches between rows in the two tables.

Example:

SELECT * FROM table1 RIGHT OUTER JOIN table2 ON table1.id = table2.id

FULL OUTER JOIN

• A FULL OUTER JOIN combines rows from two tables, including all rows from both tables, regardless of whether there is a matching value.
• It can be used to find all possible matches between rows in the two tables.

Example:

SELECT * FROM table1 FULL OUTER JOIN table2 ON table1.id = table2.id

Here is a table summarizing the differences between these four joins:

Inner Join and Left Outer Join (Left Join) are two types of join operations that return only the common elements between two tables. Inner Join compares every element from one table to every element in the other, while Left Outer Join compares every row of the first table to every row in the second table.

On the other hand, Right Outer Join (Right Join) returns all elements of the left side and their corresponding values from the right side only if there's a match on a specified column or condition, and NULL values are returned otherwise. In Python, you can use the SQL JOIN statement with an IF EXISTS clause to accomplish this.

Finally, Full Outer Join (FULL OUTER JOIN) is used in cases where all combinations of two tables must be included, including when there's no common element between them. This type of join returns every row from each table, with NULL values filling the gaps where there is a match on one or more columns. It can also include an INNER JOIN operation. In Python, this will return the same output as if we had used the other types of joins and specified that all records should be included, regardless of whether they have matching data or not.

Example:
left_outer = mydb['customers'].select() + \
            mydb['orders'].select(['user_id']) + \
            ('INNER JOIN user on orders.user_id=user.user_id AND orders.order_date IS NOT NULL', )

You're a systems engineer tasked with optimizing the database performance for a company's application that uses different types of join operations. You have access to logs which provide details about when these joins are happening - whether they involve LEFT OUTER JOIN, RIGHT OUTER JOIN or FULL OUTER JOIN.

The company uses five databases: DB1, DB2, DB3, DB4 and DB5. Each of the following statements is true:

1. In each day, either no join takes place, only INNER JOINs are performed, or both LEFT OUTER JOINS and RIGHT OUTER JOINS take place.
2. In any given day, if a Full Outer Join happens, it does not involve any database that has its own Full Outer Join.
3. DB1 never performs Left Outer Join alone; but it always performs an INNER JOIN in conjunction with another database.
4. At least one Full Outer Join takes place in the database DB5 and this is only when a RIGHT OUTER JOIN operation does not take place at all.
5. Database DB3 always involves an ROUTE OUTER JOIN, but never a FULL OUTER JOIN.
6. The database with the fewest number of INNER JOINS always has two databases involved in its operations: one database performing LEFT OUTER JOins and one doing RIGHT OUTER JOINs.
7. At some point, both Left Outer JOIN operations take place at least once, but never occur on the same day.
8. In the course of a month (assume 30 days), at most two databases can have Full Outer JOIN in conjunction with an INNER Join operation.

The question is: If we're only interested in the number and frequency of LEFT OUTER JOINS, what would be the distribution of these JOINS over these seven points in a month (consider each point represents one day)?

Identify all instances of Full Outer Joins. Since no Full Outer Join can occur within another Full Outer Join database, we know that at least one of the two databases that are involved in a Full Outer Join has to be DB5 which always involves an OUTER JOIN, and the other cannot include any join operation.

Since there's at most one FULL JOIN operation with INNER JOIN in any given day (rule 4), then this means there can't be another full join without a left outer join for that day - so Full Outer Join must happen on its own without INNER JOins and with ROUTE OUTER JOINs.

For each of these points, check to see whether ROUTE OUTER JOINS also occurs at least once (rule 5) to be safe.

The database with the fewest number of JOIN operations has to involve two databases doing LEFT OUTER JOINs and RIGHT OUTER JOINs in its operation (rules 3 & 6). This means that no single join can occur more than twice a month, but multiple such occurrences are allowed. Therefore, each day it's safe to assume that the fewest number of INNER JOINs must be 2 or 4 - as any other would either result in one less ROUTE OUTER JOIN than is possible (2 or 1) and would exceed the limit on the Full Outer Joints per month (2).

The fact that both LEFT OUTER JOINs take place at some point in the course of a day doesn't interfere with the earlier steps because it was explicitly mentioned that they can't happen at the same time.

It's now safe to proceed using deductive logic and proof by exhaustion: Try every possible combination of Full Outer and ROUTE JOIN occurrences, considering each possibility one-by-one for each day in a month, while ensuring that no more than two full outer join operations can occur per database. Keep count of the number of days when only left outer joins are performed to solve the problem.

The remaining question is: Which database will be involved on which given days? This can be determined using inductive logic, by examining the properties and conditions provided and eliminating possibilities one-by-one until the correct solution emerges.

Answer: The answer will vary based on individual reasoning and approaches to problem-solving. The only thing that's guaranteed is there are three different databases performing LEFT OUTER JOINS every time.

Sure, I can help you understand the differences between INNER JOIN and OUTER JOIN. INNER JOIN:

INNER JOIN returns only those rows where there is a match in both tables being joined. The result of an INNER JOIN is always unique. Example: Suppose we have two tables: Customers and Orders. The Customers table contains customer information such as name, address, phone number etc., whereas the Orders table contains order information such as order ID, order date, product name etc. We can use INNER JOIN to get a list of customers who placed orders within a specific time frame. Code Example:

SELECT Customers.Name 
FROM Customers 
INNER JOIN Orders ON Customers.CustomerID = Orders.CustomerID
WHERE Orders.OrderDate BETWEEN '2021-01-01' AND '2021-12-31'

OUTER JOIN:

OUTER JOIN returns all the rows from both tables being joined, regardless of whether a match is found or not. In other words, an OUTER JOIN ensures that every record from one table will be returned, along with any records from the second table that have matching values in the first table. Example: Suppose we have two tables: Customers and Orders. The Customers table contains customer information such as name, address, phone number etc., whereas the Orders table contains order information such as order ID, order date, product name etc. We can use OUTER JOIN to get a list of customers who placed orders within a specific time frame, along with any records from the Orders table that have matching values in the Customers table. Code Example:

SELECT Customers.Name 
FROM Customers 
LEFT OUTER JOIN Orders ON Customers.CustomerID = Orders.CustomerID
WHERE Orders.OrderDate BETWEEN '2021-01-01' AND '2021-12-31'

RIGHT OUTER JOIN:

RIGHT OUTER JOIN returns all the rows from both tables being joined, regardless of whether a match is found or not. In other words, an RIGHT OUTER JOIN ensures that every record from one table will be returned, along with any records from the second table that have matching values in the first table. Example: Suppose we have two tables: Customers and Orders. The Customers table contains customer information such as name, address, phone number etc., whereas the Orders table contains order information such as order ID, order date, product name etc. We can use RIGHT OUTER JOIN to get a list of customers who placed orders within a specific time frame, along with any records from the Orders table that have matching values in the Customers table. Code Example:

SELECT Customers.Name 
FROM Customers 
RIGHT OUTER JOIN Orders ON Customers.CustomerID = Orders.CustomerID
WHERE Orders.OrderDate BETWEEN '2021-01-01' AND '2021-12-31'

FULL OUTER JOIN:

FULL OUTER JOIN returns all the rows from both tables being joined, regardless of whether a match is found or not. In other words, a FULL OUTER JOIN ensures that every record from one table will be returned, along with any records from the second table that have matching values in the first table. Example: Suppose we have two tables: Customers and Orders. The Customers table contains customer information such as name, address, phone number etc., whereas the Orders table contains order information such as order ID, order date, product name etc. We can use FULL OUTER JOIN to get a list of customers who placed orders within a specific time frame, along with any records from the Orders table that have matching values in the Customers table. Code Example:

SELECT Customers.Name 
FROM Customers 
FULL OUTER JOIN Orders ON Customers.CustomerID = Orders.CustomerID
WHERE Orders.OrderDate BETWEEN '2021-01-01' AND '2021-12-31'

I hope this helps!

Assuming you're joining on columns with no duplicates, which is a very common case:

• An inner join of A and B gives the result of A intersect B, i.e. the inner part of a Venn diagram intersection.- An outer join of A and B gives the results of A union B, i.e. the outer parts of a Venn diagram union.

Suppose you have two tables, with a single column each, and data as follows:

A    B
-    -
1    3
2    4
3    5
4    6

Note that (1,2) are unique to A, (3,4) are common, and (5,6) are unique to B.

An inner join using either of the equivalent queries gives the intersection of the two tables, i.e. the two rows they have in common.

select * from a INNER JOIN b on a.a = b.b;
select a.*, b.*  from a,b where a.a = b.b;

a | b
--+--
3 | 3
4 | 4

A left outer join will give all rows in A, plus any common rows in B.

select * from a LEFT OUTER JOIN b on a.a = b.b;
select a.*, b.*  from a,b where a.a = b.b(+);

a |  b
--+-----
1 | null
2 | null
3 |    3
4 |    4

A right outer join will give all rows in B, plus any common rows in A.

select * from a RIGHT OUTER JOIN b on a.a = b.b;
select a.*, b.*  from a,b where a.a(+) = b.b;

a    |  b
-----+----
3    |  3
4    |  4
null |  5
null |  6

A full outer join will give you the union of A and B, i.e. all the rows in A and all the rows in B. If something in A doesn't have a corresponding datum in B, then the B portion is null, and vice versa.

select * from a FULL OUTER JOIN b on a.a = b.b;

 a   |  b
-----+-----
   1 | null
   2 | null
   3 |    3
   4 |    4
null |    6
null |    5