SQL: EXISTS Condition
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);