Help Center> GaussDB> User Guide> Usage Guidelines and Suggestions> Database Programming Guidelines> Subqueries
Updated on 2024-01-12 GMT+08:00
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Subqueries

  • Duplicate subquery statements are not allowed in an SQL statement.
  • Do not use scalar subqueries.

    A scalar subquery is a subquery whose result is one value and whose condition expression uses an equal operator.

    Example: non-standard statement

    SELECT * FROM t1 WHERE id = (SELECT id FROM t2 LIMIT 1);

    You are advised to split the preceding statement into two SQL statements and execute the subquery first.

  • Do not use subqueries in the SELECT target columns. Otherwise, the plan cannot be pushed down, affecting the execution performance.
  • It is recommended that the nested subqueries cannot exceed two layers.

    Subqueries cause temporary table overhead. Therefore, complex queries must be optimized based on service logic.