SQL Delete records using subqueries

SQL Deleting records with subqueries

In this page, we are going to discuss, how SUBQUERIES (A SELECT statement within another SELECT statement can be used as a subquery )along with SQL DELETE command can be used to perform a deletion.

Sample tables associated with this page have shown bellow:

Sample table: customer1

Sample table: agents

Sample table: customer

Sample table: agent1

Sample table: orders

Example:

To remove rows from the table 'customer1' with following conditions -

1. 'agent_code' should be any 'agent_code' from 'agents' table which satisfies the condition bellow :

2. 'working_area' of 'agents' table must be 'London',

the following SQL statement can be used:

SQL Code:

DELETE FROM customer1
WHERE agent_code=ANY(
SELECT agent_code FROM agents
WHERE working_area='London');

Output:

SQL delete records using subqueries with alias

In this page, we are going to discuss, how table aliases( when two or more tables used in a query, then alias makes it easy to read and write with a short name which comes after the table name after the FROM keyword) can be used with SUBQUERIES (A SELECT statement within another SELECT statement can be used as a subquery ), and with the help of subqueries SQL DELETE command can be used to delete records.

Example:

To remove rows from the table 'agent1' with following conditions -

1. 'da' and 'cu' are the aliases for the table 'agent1' and 'customer'

2. check the existence of the subquery is true or false. which satisfies the condition bellow :

3. 'grade' of 'customer' table must be 3,

4. 'agent_code' of 'agent1' table and 'agent_code' of 'customer' table should not be same,

the following SQL statement can be used:

SQL Code:

DELETE FROM agent1 da
WHERE EXISTS(
SELECT * FROM  customer cu
WHERE grade=3
AND da.agent_code<>cu.agent_code);

Output:

SQL delete records using subqueries with alias and IN

In this page we are going to discuss, how rows can be removed from a table by SQL DELETE statement with the use of IN operator and SUBQUERIES.

Example:

To remove rows from the table 'agent1' with following conditions -

1. 'da' and 'cu' are the aliases of 'agent1' and 'customer' table,

2. check the number 3 is in the result of the subquery which satisfies the condition bellow :

3. 'agent_code' of 'agent1' table and 'agent_code' of 'customer' table should not be same,

the following SQL statement can be used:

SQL Code:

DELETE FROM agent1 da
WHERE 3 IN(
SELECT grade FROM customer cu
WHERE agent1.agent_code<>customer.agent_code);

Output:

SQL delete records using subqueries with alias and MIN

In this page, we are going to discuss, how rows can be removed from a table by SQL DELETE statement along with the SQL MIN() function.

Example:

To remove rows from the table 'agent1' with following conditions -

1. 'orders' table used as alias 'a' and alias 'b',

2. 'agent_code' of 'agent1' should be within the 'agent_code' in alias 'a' which satisfies the condition bellow :

i) 'ord_amount' of alias 'a' must be equal to the minimum 'ord_amount' of alias 'b' which satisfies the condition bellow :

a) 'ord_date' of alias 'a' and alias 'b' must be equal,

the following SQL statement can be used :

SQL Code:

DELETE FROM agent1
WHERE agent_code IN
(SELECT agent_code FROM orders a
WHERE ord_amount=(
SELECT MIN(ord_amount) FROM orders b
WHERE a.ord_date=b.ord_date));

Output:

SQL delete records using subqueries with alias and MIN and COUNT

In this page, we are going to discuss, how rows can be removed from a table by SQL DELETE statement along with the SQL MIN() and COUNT() function.

Example:

To remove rows from the table 'agent1' with following conditions -

1. 'orders' table used as alias 'a' and alias 'b'

2. 'agent_code' of 'agent1' should be within the 'agent_code' in alias 'a' which satisfies the condition bellow:

i) 'ord_amount' of alias 'a' must be equal to the minimum 'ord_amount' of alias 'b' which satisfies the condition bellow :

a) 'ord_date' of alias 'a' and alias 'b' must be equal

ii) the number 1 should be less than the number of 'ord_num' form alias 'b' which satisfies the condition bellow :

a) 'ord_date' of alias 'a' and alias 'b' must be equal,

the following SQL statement can be used :

SQL Code:

DELETE FROM agent1
WHERE agent_code IN(
SELECT agent_code FROM orders a
WHERE ord_amount=(
SELECT MIN(ord_amount) FROM orders b
WHERE a.ord_date=b.ord_date)
AND 1<(
SELECT COUNT(ord_num) FROM orders b
WHERE a.ord_date=b.ord_date));

Output:

Outputs of the said SQL statement shown here is taken by using Oracle Database 10g Express Edition.

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