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03.12.1999 at 10:36PM PST, ID: 10138459
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SQL EXIST
Tags: sql, exist
Access documentation seems very poor in regard to SQL SELECT. Can someone explain the use of EXISTS in a WHERE clause?
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Question Stats
Zone: Microsoft
Question Asked By: ramrom
Solution Provided By: vendrig
Participating Experts: 2
Solution Grade: A
Views: 793
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03.13.1999 at 05:27AM PST, ID: 1982746

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03.13.1999 at 09:54AM PST, ID: 1982747

Rank: Wizard

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03.13.1999 at 10:00AM PST, ID: 1982748

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03.13.1999 at 10:03AM PST, ID: 1982749

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03.13.1999 at 10:07AM PST, ID: 1982750

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03.13.1999 at 10:34AM PST, ID: 1982751

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03.13.1999 at 05:27AM PST, ID: 1982746
It's for inner joins. Exists determines if objects are present in a table. E.g.
SELECT * FROM tabA
WHERE EXISTS(SELECT * from tabB where tabA.ID=tabB.ID);

Gives all information about every record in A that has a related record in B (determined by tabA.empID=tabB.empID)
 
03.13.1999 at 09:54AM PST, ID: 1982747

Rank: Wizard

My understanding of Exists: Exists returns true if the provided SQL statement returns one or more records. You will most likely use it to check if there are records present in another table that match a criteria (same ContractNo, CountryCode as your current record/task ...) Sub queries are a bit more complicated than just linking in the other table, but can give you more control and other opportunities. So it is not for INNER JOINS, but more like ANY KIND OF JOIN.

Have a look in Access Help under SQL Subqueries. You will find examples of Exists, All and Any that are operators that can be used on Sub Queries.


I will typical use Exists in triggers in a SQL server database to ensure relationship integrity.

Example: ContractTable and ContractDetailsTable. Now in the delete trigger for the ContractTable I will put a
If Exists(SELECT * FROM ContractDetailsTable, Deleted WHERE ContractID = Deleted.ContractID) Goto Rollback

...

Rollback:

  RAISERROR ('---- Record coulde not be deleted due to relational integrity issues ----',18,1) Rollback TRANSACTION
 
03.13.1999 at 10:00AM PST, ID: 1982748
vendrig's comment is the answer I wish to accept. Please re-post it as an answer.
 
03.13.1999 at 10:03AM PST, ID: 1982749
It's for inner joins. Exists determines if objects are present in a table. E.g.
SELECT * FROM tabA
WHERE EXISTS(SELECT * from tabB where tabA.ID=tabB.ID);

Gives all information about every record in A that has a related record in B (determined by tabA.empID=tabB.empID)

Accepted Solution
 
03.13.1999 at 10:07AM PST, ID: 1982750
That was FAST! And so is this.
 
03.13.1999 at 10:34AM PST, ID: 1982751

Rank: Wizard

Note that Exists is not for INNER JOINS, it is an operator used on subqueries, which may be considered as a kind of join.
 
 
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