How Do Venn Diagrams Illustrate Different Types of SQL Joins?
Jan 14, 2025 am 09:07 AMUsing 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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