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Editorial Team
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Asked: 2026-05-13T18:01:42+00:00

Hello I made a SQL test and dubious/curious about one question: In which sequence

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Hello I made a SQL test and dubious/curious about one question:

In which sequence are queries and sub-queries executed by the SQL engine?

the answers was

  1. primary query -> sub query -> sub sub query and so on
  2. sub sub query -> sub query -> prime query
  3. the whole query is interpreted at one time
  4. There is no fixed sequence of interpretation, the query parser takes a decision on fly

I choosed the last answer (just supposing that it is most reliable w.r.t. others).
Now the curiosity:

where can i read about this and briefly what is the mechanism under all of that?

Thank you.

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1 Answer

  1. Editorial Team

    Option 4 is close.

    SQL is declarative: you tell the query optimiser what you want and it works out the best (subject to time/”cost” etc) way of doing it. This may vary for outwardly identical queries and tables depending on statistics, data distribution, row counts, parallelism and god knows what else.

    This means there is no fixed order. But it’s not quite “on the fly”

    Even with identical servers, schema, queries, and data I’ve seen execution plans differ

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