An SQL join clause combines records from two or more tables in a database. ANSI standard SQL specifies four types of JOINs: INNER, OUTER, LEFT, and RIGHT. Since SQL joins appear to be set-based, its easy to explain using Venn diagrams.
LEFT OUTER JOIN: produces a complete set of records from Table A, with the matching records (where available) in Table B. If there is no match, the right side will contain null.
RIGHT OUTER JOIN: produces a complete set of records from Table B, with the matching records (where available) in Table A. If there is no match, the left side will contain null.
FULL OUTER JOIN: produces the set of all records in Table A and Table B, with matching records from both sides where available. If there is no match, the missing side will contain null.
INNER JOIN: produces only the set of records that match in both Table A and Table B.
8 thoughts on “Visual Explanation of SQL Joins”
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