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sqlsql-server-2008coalesce

Understanding the Syntax for COALESCE

I am trying to understand the below syntax. Can I get some help with this.

DECLARE @StringList VARCHAR(2500); 
SELECT  COALESCE(@StringList + ',','') + CAST(apID as VARCHAR) AS ApIdList FROM testTable
Solution

  • As a result you will get all apID from testTable in VARCHAR

    COALESCE checks if first parameter is NULL then the second parameter will returned. In this line @StringList is always equals NULL

    COALESCE(@StringList + ',','')

    So, NULL + ',' = NULL and you will get empty string ('')

    Then empty string + CAST(apID as VARCHAR) and you will get apID as VARCHAR