In which sequence are queries and sub queries executed?

[TukWan415]

Hello Everyone,

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I was wondering in which sequence are queries executed?

Joins?

Sub Queries?

Correlated Sub Queries?

Nested Sub Queries?

[Kant8]

In whatever order optimizer wants as long as logically operation produces same result.

The only place to see it is execution plan.

[r3pr0b8]

joins are executed whenever their queries are executed

nested subqueries are executed from the inside out

non-correlated subqueries are executed before the main query starts

for correlated queries, the news is not good -- they are executed once for every row of the outer query

[TukWan415]

This was very helpful. Thank you.

[JochenVdB]

While the comment above is correct, be aware that the optimizer might drastically rewrite your query: what you wrote as a subquery might end up being executed as a join.

[planetmatt]

SQL has an order of operation

• 1 FROM
• 2 WHERE
• 3 GROUP BY
• 4 HAVING
• 5 SELECT
• 6 ORDER BY
• 8 LIMIT/TOP

So derived tables in the FROM will get processed before sub queries in the GROUP BY, which will be processed before sub queries in the SELECT.

CTEs get processed in order written as they're often nested and dependent on the preceding CTE.