SQL UNION clause

SQL UNION clause

Sql UNION clauses/operators are used to combine the results of two or more SELECT statements.

When using UNION, each SELECT statement must have the same number of selected columns, the same number of column expressions, the same data type, and they appear in the same order, but not necessarily the same length.

Grammar

The basic syntax of the UNION clause is as follows:

    SELECT column1 [, column2 ]
    FROM table1 [, table2 ]
    [WHERE condition]

    UNION

    SELECT column1 [, column2 ]
    FROM table1 [, table2 ]
    [WHERE condition]

The conditions here can be any condition that is tailored to your needs.

Example

Consider the following two tables, (a) the CUSTOMERS table:

    +----+----------+-----+-----------+----------+
    | ID | NAME     | AGE | ADDRESS   | SALARY   |
    +----+----------+-----+-----------+----------+
    |  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
    |  2 | Khilan   |  25 | Delhi     |  1500.00 |
    |  3 | kaushik  |  23 | Kota      |  2000.00 |
    |  4 | Chaitali |  25 | Mumbai    |  6500.00 |
    |  5 | Hardik   |  27 | Bhopal    |  8500.00 |
    |  6 | Komal    |  22 | MP        |  4500.00 |
    |  7 | Muffy    |  24 | Indore    | 10000.00 |
    +----+----------+-----+-----------+----------+

(b) The other table is the ORDERS table, as follows:

    +-----+---------------------+-------------+--------+
    |OID  | DATE                | CUSTOMER_ID | AMOUNT |
    +-----+---------------------+-------------+--------+
    | 102 | 2009-10-08 00:00:00 |           3 |   3000 |
    | 100 | 2009-10-08 00:00:00 |           3 |   1500 |
    | 101 | 2009-11-20 00:00:00 |           2 |   1560 |
    | 103 | 2008-05-20 00:00:00 |           4 |   2060 |
    +-----+---------------------+-------------+--------+

Now, let's connect the two tables with select statements:

    SQL> SELECT  ID, NAME, AMOUNT, DATE
         FROM CUSTOMERS
         LEFT JOIN ORDERS
         ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID
    UNION
         SELECT  ID, NAME, AMOUNT, DATE
         FROM CUSTOMERS
         RIGHT JOIN ORDERS
         ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;

The result is as follows:

    +------+----------+--------+---------------------+
    | ID   | NAME     | AMOUNT | DATE                |
    +------+----------+--------+---------------------+
    |    1 | Ramesh   |   NULL | NULL                |
    |    2 | Khilan   |   1560 | 2009-11-20 00:00:00 |
    |    3 | kaushik  |   3000 | 2009-10-08 00:00:00 |
    |    3 | kaushik  |   1500 | 2009-10-08 00:00:00 |
    |    4 | Chaitali |   2060 | 2008-05-20 00:00:00 |
    |    5 | Hardik   |   NULL | NULL                |
    |    6 | Komal    |   NULL | NULL                |
    |    7 | Muffy    |   NULL | NULL                |
    +------+----------+--------+---------------------+

UNION ALL clause:

The UNION ALL operator is used to combine the results of two SELECT statements, and duplicate rows are included.

The UNION ALL operator follows the same rules as union.

Grammar:

The basic syntax of UNION ALL is as follows:

    SELECT column1 [, column2 ]
    FROM table1 [, table2 ]
    [WHERE condition]

    UNION ALL

    SELECT column1 [, column2 ]
    FROM table1 [, table2 ]
    [WHERE condition]

Example:

Consider the following two tables, (a) the CUSTOMERS table:

    +----+----------+-----+-----------+----------+
    | ID | NAME     | AGE | ADDRESS   | SALARY   |
    +----+----------+-----+-----------+----------+
    |  1 | Ramesh   |  32 | Ahmedabad |  2000.00 |
    |  2 | Khilan   |  25 | Delhi     |  1500.00 |
    |  3 | kaushik  |  23 | Kota      |  2000.00 |
    |  4 | Chaitali |  25 | Mumbai    |  6500.00 |
    |  5 | Hardik   |  27 | Bhopal    |  8500.00 |
    |  6 | Komal    |  22 | MP        |  4500.00 |
    |  7 | Muffy    |  24 | Indore    | 10000.00 |
    +----+----------+-----+-----------+----------+

(b) The other table is the ORDERS table, as follows:

    +-----+---------------------+-------------+--------+
    |OID  | DATE                | CUSTOMER_ID | AMOUNT |
    +-----+---------------------+-------------+--------+
    | 102 | 2009-10-08 00:00:00 |           3 |   3000 |
    | 100 | 2009-10-08 00:00:00 |           3 |   1500 |
    | 101 | 2009-11-20 00:00:00 |           2 |   1560 |
    | 103 | 2008-05-20 00:00:00 |           4 |   2060 |
    +-----+---------------------+-------------+--------+

Now, let's connect the two tables with select statements:

SQL> SELECT  ID, NAME, AMOUNT, DATE
     FROM CUSTOMERS
     LEFT JOIN ORDERS
     ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID
UNION ALL
     SELECT  ID, NAME, AMOUNT, DATE
     FROM CUSTOMERS
     RIGHT JOIN ORDERS
     ON CUSTOMERS.ID = ORDERS.CUSTOMER_ID;

The result is as follows:

    +------+----------+--------+---------------------+
    | ID   | NAME     | AMOUNT | DATE                |
    +------+----------+--------+---------------------+
    |    1 | Ramesh   |   NULL | NULL                |
    |    2 | Khilan   |   1560 | 2009-11-20 00:00:00 |
    |    3 | kaushik  |   3000 | 2009-10-08 00:00:00 |
    |    3 | kaushik  |   1500 | 2009-10-08 00:00:00 |
    |    4 | Chaitali |   2060 | 2008-05-20 00:00:00 |
    |    5 | Hardik   |   NULL | NULL                |
    |    6 | Komal    |   NULL | NULL                |
    |    7 | Muffy    |   NULL | NULL                |
    |    3 | kaushik  |   3000 | 2009-10-08 00:00:00 |
    |    3 | kaushik  |   1500 | 2009-10-08 00:00:00 |
    |    2 | Khilan   |   1560 | 2009-11-20 00:00:00 |
    |    4 | Chaitali |   2060 | 2008-05-20 00:00:00 |
    +------+----------+--------+---------------------+

In addition, there are two clauses (i.e. computational) that are very similar to union clauses:

• SQL INTERSECT clause: Used to combine two SELECT statements, but returns only the lines that are in the results of both SELECT statements.
• SQL EXCEPT clause: Combines two SELECT statements and returns rows that exist in the results of the first SELECT statement, but do not exist in the results of the second SELECT statement.