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Old October 12th, 2006, 09:48 AM
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Default difference between 2 tables


Have 2 different tables(table A, table B) with a list of users. Now I would like to list all the users in table A that are NOT in table B. How would one make the general SQL query for that?

Thank you,

Old October 12th, 2006, 09:56 AM
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select table1.user from table1 left join table2 on table1.user = table2.user where table2.user is null

Old October 12th, 2006, 09:57 AM
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There are two ways.

First, you can use a subquery to find all the users in the first table NOT IN the second, as:
or, being trickier, you can use an OUTER JOIN of the two tables and select the User from the TableB columns of the result where the User is NULL:
SELECT TableA.User
  FROM TableA
  LEFT OUTER JOIN TableB ON TableA.User = TableB.User
admittedly, the second query is harder to understand.

It's not clear which is more efficient. Indeed, they may result in the same execution plan. Or not.

Jeff Mason
Custom Apps, Inc.

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