EXISTS Condition

SQL: EXISTS Condition

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The EXISTS condition is considered "to be met" if the subquery returns at least one row.
The syntax for the EXISTS condition is:

SELECT columns
FROM tables
WHERE EXISTS ( subquery );

The EXISTS condition can be used in any valid SQL statement - select, insert, update, or delete.

Note:

These EXISTS queries are very inefficient since the sub-query is RE-RUN for EVERY row in the outer query's table. There are more efficient ways to write most queries, that do not use Exists.

Example #1:

Let's take a look at a simple example. The following is an SQL statement that uses the EXISTS condition:

SELECT *
FROM suppliers
WHERE EXISTS
  (select *
    from orders
    where suppliers.supplier_id = orders.supplier_id);

This select statement will return all records from the suppliers table where there is at least one record in the orders table with the same supplier_id.

Example #2 - NOT EXISTS:

The EXISTS condition can also be combined with the NOT operator.
For example,

SELECT *
FROM suppliers
WHERE not exists (select * from orders Where suppliers.supplier_id = orders.supplier_id);

This will return all records from the suppliers table where there are no records in the orders table for the given supplier_id.

Example #3 - DELETE Statement:

The following is an example of a delete statement that utilizes the EXISTS condition:

DELETE FROM suppliers
WHERE EXISTS
  (select *
    from orders
    where suppliers.supplier_id = orders.supplier_id);

Example #4 - UPDATE Statement:

The following is an example of an update statement that utilizes the EXISTS condition:

UPDATE suppliers
SET supplier_name =	( SELECT customers.name FROM customers WHERE customers.customer_id = suppliers.supplier_id)
WHERE EXISTS   ( SELECT customers.name     FROM customers     WHERE customers.customer_id = suppliers.supplier_id);

Example #5 - INSERT Statement:

The following is an example of an insert statement that utilizes the EXISTS condition:

INSERT INTO suppliers
(supplier_id, supplier_name)
SELECT account_no, name
FROM suppliers
WHERE exists (select * from orders Where suppliers.supplier_id = orders.supplier_id);