Sabtu, 27 September 2014
Description
The SQL EXISTS condition is used in
combination with a subquery and is considered to be met, if the subquery
returns at least one row. It can be used in a SELECT, INSERT, UPDATE,
or DELETE statement.
Syntax
The syntax for the SQL EXISTS condition is:WHERE EXISTS ( subquery );
Parameters or Arguments
subquery is a SELECT statement.Note
- SQL Statements that use the SQL EXISTS Condition 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 the SQL EXISTS Condition.
Example - With SELECT Statement
Let's look at a simple example.The following is a SQL SELECT statement that uses the SQL EXISTS condition:
SELECT *
FROM suppliers
WHERE EXISTS (SELECT *
FROM orders
WHERE suppliers.supplier_id = orders.supplier_id);
This SQL EXISTS condition example 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 - With SELECT Statement using 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 SQL EXISTS example will return all records from the suppliers table where there are no records in the orders table for the given supplier_id.
Example - With INSERT Statement
The following is an example of a SQL INSERT statement that uses the SQL EXISTS condition :INSERT INTO contacts
(contact_id, contact_name)
SELECT supplier_id, supplier_name
FROM suppliers
WHERE EXISTS (SELECT *
FROM orders
WHERE suppliers.supplier_id = orders.supplier_id);
Example - With UPDATE Statement
The following is an example of a SQL UPDATE statement that uses the SQL 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 - With DELETE Statement
The following is an example of a SQL DELETE statement that uses the SQL EXISTS condition :DELETE FROM suppliers
WHERE EXISTS (SELECT *
FROM orders
WHERE suppliers.supplier_id = orders.supplier_id);
Source : techonthenet.com
