Relational algebra and SQL

Vocabulary

English	Meaning
selection/sɪˈlekʃn/	Choosing instructions by a condition; in relational algebra, choosing rows that satisfy a condition.
projection/prəˈdʒekʃn/	A relational-algebra operation choosing specified columns from a relation.
join/dʒɔɪn/	An SQL operation that combines rows from tables according to a specified relationship or condition.

Five operators, one pipeline

The algebra: selection σ keeps rows, projection π keeps columns, join ⋈ combines on a condition, union and difference combine tables. Every SELECT is these five in a pipeline.

{exercise:uni-cs0802-algebra}

Practice

The operator that keeps columns is —

Difficulty 1/5 · 100 coins + a small bonus on first completion today

FROM builds the row set, WHERE filters rows, GROUP BY partitions, HAVING filters groups, SELECT projects, ORDER BY sorts. Filtering an aggregate in WHERE is a category error the engine rejects — use HAVING.
The course query: join Course and Enrollment, select credits = 15, group by title, keep groups of size $\ge 2$. Executed on the sample schema the result is Databases | 3.

{exercise:uni-cs0802-order}

Practice

Filtering on COUNT(*) happens in —

Difficulty 1/5 · 100 coins + a small bonus on first completion today

A plain join drops the 2 courses with no enrollment; LEFT JOIN keeps them with n = 0 — 6 rows. COUNT(*) counts rows; COUNT(col) skips NULLs.
Projecting a many-side column repeats values: 14 enrollment rows project 14 student ids — add DISTINCT when you mean the set.

Example

Read every query as a pipeline and the output stops being surprising.

{exercise:uni-cs0802-join}

Practice

The course query on the sample schema returns —

Difficulty 1/5 · 100 coins + a small bonus on first completion today