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SQL Server FAQ - EXISTS - Testing Subquery Results

By: FYIcenter.com

(Continued from previous topic...)

How To Test Subquery Results with the EXISTS Operator?

EXISTS is a special operator used to test subquery results. EXISTS can be used in two ways:

EXISTS (SELECT ...)
-- Returns TRUE if the specified subquery has one or more rows returned. 

NOT EXISTS (SELECT ...)
-- Returns TRUE if the specified subquery no rows returned. 

The following tutorial exercise shows you two examples of EXISTS operators. The sample database AdventureWorksLT provided by Microsoft is used.

USE AdventureWorksLT
GO

-- Number of customers with orders
SELECT COUNT(*) FROM SalesLT.Customer c
WHERE EXISTS (
   SELECT * FROM SalesLT.SalesOrderHeader s
   WHERE s.CustomerID = c.CustomerID
   )
GO
32

-- Number of customers without orders
SELECT COUNT(*) FROM SalesLT.Customer c
WHERE NOT EXISTS (
   SELECT * FROM SalesLT.SalesOrderHeader s
   WHERE s.CustomerID = c.CustomerID
   )
408

(Continued on next topic...)

  1. What Is a Boolean Value?
  2. What Are Conditional Expressions?
  3. What Are Comparison Operations?
  4. How To Perform Comparison on Exact Numbers?
  5. How To Perform Comparison on Floating Point Numbers?
  6. How To Perform Comparison on Date and Time Values?
  7. How To Perform Comparison on Character Strings?
  8. What To Test Value Ranges with the BETWEEN Operator?
  9. What To Test Value Lists with the IN Operator?
  10. What To Perform Pattern Match with the LIKE Operator?
  11. How To Use Wildcard Characters in LIKE Operations?
  12. How To Test Subquery Results with the EXISTS Operator?
  13. How To Test Values Returned by a Subquery with the IN Operator?
  14. What Are Logical/Boolean Operations?

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