Parentheses are sometimes called precedence operators - this means that they
can be used to change the other operator's precedence in an expression. The
expressions that are written between parentheses are computed before the
expressions that are written outside. Parentheses must always contain an
expression (that is, they cannot be empty), and can be nested.

For example, the following expressions could return different results:

* NOT a OR b
* NOT (a OR b)

In the first case, NOT applies to a, so if a is FALSE or b is TRUE, the
expression returns TRUE. In the second case, NOT applies to the result of a OR
b, so if at least one of a or b is TRUE, the expression is TRUE.

When the precedence of operators is not intuitive, you can use parentheses to
make it immediately clear for whoever reads the statement.

The precedence of the NOT operator can also be affected by the
HIGH_NOT_PRECEDENCE SQL_MODE flag.

Other uses
----------

Parentheses must always be used to enclose subqueries.

Parentheses can also be used in a JOIN statement between multiple tables to
determine which tables must be joined first.

Also, parentheses are used to enclose the list of parameters to be passed to
built-in functions, user-defined functions and stored routines. However, when
no parameter is passed to a stored procedure, parentheses are optional. For
builtin functions and user-defined functions, spaces are not allowed between
the function name and the open parenthesis, unless the IGNORE_SPACE SQL_MODE
is set. For stored routines (and for functions if IGNORE_SPACE is set) spaces
are allowed before the open parenthesis, including tab characters and new line
characters.

Syntax errors
-------------

If there are more open parentheses than closed parentheses, the error usually
looks like this:

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MariaDB server version for the right syntax to use near ''
a
t line 1

Note the empty string.

If there are more closed parentheses than open parentheses, the error usually
looks like this:

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MariaDB server version for the right syntax to use near ')'
at line 1

Note the quoted closed parenthesis.

URL: https://mariadb.com/kb/en/parentheses/