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SQL Server FAQ - What Is a Subquery in a SELECT Query Statement

By: FYIcenter.com

(Continued from previous topic...)

What Is a Subquery in a SELECT Query Statement?

A subquery is a SELECT statement used as part of the selection criteria of the main SELECT statement. The subquery specified in the WHERE clause will be evaluated repeated on each row of the selection base table. The output of the subquery will be used in the final evaluation of the criteria. Usually, subqueries are used in the following Boolean operations:

  • "expression IN (subquery)" - True if the expression matches one of the returned values from the subquery.
  • "expression NOT IN (subquery)" - True if the expression does not match any of the returned values from the subquery.
  • "EXISTS (subquery)" - True if the subquery returns one or more rows.
  • "NOT EXISTS (subquery)" - True if the subquery returns no rows.

(Continued on next topic...)

  1. How To Join Two Tables in a Single Query?
  2. How To Write a Query with an Inner Join?
  3. How To Define and Use Table Alias Names?
  4. How To Write a Query with a Left Outer Join?
  5. How To Write a Query with a Right Outer Join?
  6. How To Write a Query with a Full Outer Join?
  7. How To Write an Inner Join with the WHERE Clause?
  8. How To Name Query Output Columns?
  9. What Is a Subquery in a SELECT Query Statement?
  10. How To Use Subqueries with the IN Operators?
  11. How To Use Subqueries with the EXISTS Operators?
  12. How To Use Subqueries in the FROM Clause?
  13. How To Count Groups Returned with the GROUP BY Clause?
  14. How To Return the Top 5 Rows from a SELECT Query?
  15. How To Return the Second 5 Rows?
  16. How To Use UNION to Merge Outputs from Two Queries Together?
  17. How To Use ORDER BY with UNION Operators

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