There are situations when we need to combine the results from two or more SELECT statements. SQL enables us to handle these requirements by using set operations. The result of each SELECT statement can be treated as a set, and SQL set operations can be applied on those sets to arrive at a final result. Oracle SQL supports the following four set operations:

• UNION ALL
• UNION
• MINUS
• INTERSECT

SQL statements containing these set operators are referred to as compound queries, and each SELECT statement in a compound query is referred to as a component query. Two SELECTs can be combined into a compound query by a set operation only if they satisfy the following two conditions:

• The result sets of both the queries must have the same number of columns.
• The data type of each column in the second result set must match the data type of its corresponding column in the first result set.

These conditions are also referred to as union compatibility conditions. The term union compatibility is used even though these conditions apply to other set operations as well. Set operations are often called vertical joins, because the result combines data from two or more SELECTS based on columns instead of rows. The generic syntax of a query involving a set operation is:

component_query

{UNION | UNION ALL | MINUS | INTERSECT}

component_query

The keywords UNION, UNION ALL, MINUS, and INTERSECT are set operators. You can have more than two component queries in a composite query; you will always use one less set operator than the number of component queries.

There is an exception to the second union compatibility condition. Two data types do not need to be the same if they are in the same data type group. By data type group, we mean the general categories such as numbers, strings, and datetimes. For example, it is ok to have a column in the first component query of data type CHAR, that corresponds to a VARCHAR2 column in the second component query (or vice versa). Oracle performs implicit type conversion in such a case.

However, Oracle will not perform implicit type conversion if corresponding columns in the component queries belong to different data type groups. For example, if a column in the first component query is of data type DATE, and the corresponding column in the second component query is of data type CHAR, Oracle will not perform implicit conversion, and you will get an error as a result of violation of data type compatibility. This is illustrated in the following example:

SELECT TO_DATE('12-OCT-03') FROM DUAL

UNION

SELECT '13-OCT-03' FROM DUAL;

SELECT TO_DATE('12-OCT-03') FROM DUAL

*

ERROR at line 1:

ORA-01790: expression must have same datatype as corresponding expression

The following sections discuss syntax, examples, rules, and restrictions for the four set operations.