How Do Venn Diagrams Illustrate Different Types of SQL Joins?

Using Venn diagram to illustrate SQL connection types

Venn diagram is an effective visual representation of SQL connections, helping users understand how data from different tables is combined. Here is a summary of the different connection types represented using a modified Venn diagram:

Clockwise from the upper right corner:

1. Inner join (circle B is colored red, overlapping parts are colored green)

• Purpose: Return only rows where there is a match between the two tables.
• SQL statement: SELECT A.Colour, B.Colour FROM A INNER JOIN B ON A.Colour = B.Colour
• Explanation: The SQL statement prioritizes table B, placing it before the "ON" clause, but the resulting rows focus on the data shared between the two tables.

2. Inner join (only contains B circles, overlapping parts are colored green)

• Purpose: Return only rows in circle B that are connected to any row in circle A (in this case, there are none).
• SQL statement: SELECT A.Colour, B.Colour FROM A INNER JOIN B ON A.Colour NOT IN ('Green','Blue')
• Explanation: The join condition excludes certain values ​​from the A circles, effectively limiting the resulting rows to the B circles.

3. Cross connection (all data included)

• Purpose: Combine every row in circle A with every row in circle B, regardless of whether there is overlap or matching.
• SQL statement: SELECT A.Colour, B.Colour FROM A CROSS JOIN B
• Explanation: The resulting number of rows is the product of the number of rows in each table (in this case 4x4 = 16).

4. Left outer join (including A circle, overlapping parts colored green)

• Purpose: Contains all rows in circle A (regardless of overlap or not), and connects matching rows in circle B.
• SQL statement: SELECT A.Colour, B.Colour FROM A LEFT OUTER JOIN B ON A.Colour = B.Colour
• Instructions: Keep all unmatched rows in circle A and use NULL values ​​in the columns of circle B.

5. Left outer join (excluding circle A, overlapping parts are colored green)

• Purpose: Similar to the previous join, but only includes rows in circle A that are not connected to any rows in circle B.
• SQL statement: SELECT A.Colour, B.Colour FROM A LEFT OUTER JOIN B ON A.Colour = B.Colour WHERE B.Colour IS NULL
• Instructions: Isolate the unmatched rows in the A circle, producing separate red rows in this case.

6. Right outer join (including B circle, the overlapping part is colored green)

• Purpose: Similar to a left outer join, but retains unmatched rows in circle B and uses NULL values ​​in the columns of circle A.
• SQL statement: SELECT A.Colour, B.Colour FROM A RIGHT OUTER JOIN B ON A.Colour = B.Colour
• Note: In this example only the blue rows in circle B are included because there are no matches in circle A.

7. Full outer join (all data included)

• Purpose: Combine all the rows in circle A and circle B, keep the non-matching rows, and fill the columns in the non-matching rows with NULL values.
• SQL statement: SELECT A.Colour, B.Colour FROM A FULL OUTER JOIN B ON A.Colour = B.Colour
• Note: The result row includes two circles, and unmatched rows are expanded with NULL values.

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