How do you combine the results of two separate queries?
Oracle provides three commands,
that handle combining query results in three different ways.
The most commonly used command, UNION combines the two answer sets into a single answer set. It
automatically removes duplicate rows from the results.
INTERSECT gives you the rows that are found in both queries by eliminating rows that are only found in one
or the other query.
MINUS gives you the rows that are found in the first query and not in the second query by removing from the
results all the rows that are found only in the second query.
UNION, INTERSECT, and MINUS SQL Operations
Differences between commands
The following animation shows you the difference between these three commands by using two circles to represent two query result sets, labeled
A and B. The animation shows which portions of the two result sets are returned by combining the two queries with each of the three set
commands: INTERSECT, then UNION, then MINUS.
There are two important rules to follow when using these commands:
Both queries must have matching lists of columns with matching datatypes. In other words, if your first query returns three columns, a
date, a number, and a character column, your second query must also return three columns: a date, a number, and a character column, in
that order. The two queries do not need to use the same column names.
The first query's list of columns is used for the final results set. If two corresponding columns in the two queries have different
lengths, the first query is used to set the length of the final results set. If the second query's column is longer, you may get an
error in your results due to data truncation.
Here is an example of two queries connected with the INTERSECT command:
ORDER BY 1
This query finds the dates (year and month only) and the employee names that are the same for changes made to the PRODUCT table and log entries made to the PET_CARE_LOG.
In the next lesson, you will learn how to use advanced set operators.