Database Reference
In-Depth Information
17
In this chapter you learn how to use the
UNION
operator to combine multiple
SELECT
statements into one result set.
Most SQL queries contain a single
SELECT
statement that returns data from
one or more tables. MariaDB also enables you to perform multiple queries
(multiple
SELECT
statements) and return the results as a single query result set.
These combined queries are usually known as
unions
or
compound queries
.
There are basically two scenarios in which you'd use combined queries:
To return similarly structured data from different tables in a single
query
■
To perform multiple queries against a single table returning the data as
one query
■
Tip
Combining Queries and Multiple
WHERE
Conditions For the most part, combining two
queries to the same table accomplishes the same thing as a single query with multiple
WHERE
clause conditions. In other words, any
SELECT
statement with multiple
WHERE
clauses can also be specified as a combined query, as you see in the section that fol-
lows. However, the performance of each of the two techniques can vary based on the
queries used. As such, it is always good to experiment to determine which is preferable
for specific queries.
SQL queries
are combined using the
UNION
operator. Using
UNION
, multiple
SELECT
statements can be specified, and their results can be combined into a
single result set.
