SQL Query Order

SQL is written SELECT-first but evaluated FROM-first. Follow the rows through every clause in logical order and the rules about aliases and aggregates stop being arbitrary.

9 clauses

One table in, one table out

Aliases born in SELECT

Query

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What Each Clause Can See

Clause	Aggregates	Aliases
WHERE	no	no
GROUP BY	no	no*
HAVING	yes	no*
SELECT	yes	defines them
ORDER BY	yes	yes

* MySQL and PostgreSQL accept SELECT aliases in GROUP BY (and MySQL in HAVING) as an extension; standard SQL does not. The optimizer may reorder the physical work — pushing filters down, for instance — but never the logical result.

Knowledge Check

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Why does WHERE COUNT(*) > 2 fail, while HAVING COUNT(*) > 2 works?

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SQL Query Order

What Each Clause Can See

Knowledge Check

See also

SQL Query Order

What Each Clause Can See

Knowledge Check

See also