SQL: JOIN Operations

When data is split across multiple normalised tables, you use JOINs to combine rows from two or more tables based on a related column (usually a foreign key relationship). Joins are one of the most powerful features of SQL and a core topic at A-Level.

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Why Do We Need Joins?

After normalisation, data that logically belongs together is stored in separate tables to avoid redundancy. To view this data together, we must join the tables.

Example Tables:

Students

StudentID	FirstName	Surname	FormGroup
1	Alice	Chen	10A
2	Bob	Patel	10B
3	Carol	Smith	10A

Grades

GradeID	StudentID	Subject	Score
1	1	Maths	85
2	1	English	72
3	2	Maths	91
4	4	Science	68

Note that GradeID 4 references StudentID 4, who does not exist in the Students table (this will illustrate differences between join types).

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INNER JOIN