24.3. Expression Handling
With precision math, exact-value numbers are used as given
whenever possible. For example, numbers in comparisons are used
exactly as given without a change in value. In strict SQL mode,
for INSERT into a column with an exact data
type (DECIMAL or integer), a number is inserted
with its exact value if it is within the column range. When
retrieved, the value should be the same as what was inserted.
(Without strict mode, truncation for INSERT is
allowable.)
Handling of a numeric expression depends on what kind of values
the expression contains:
If any approximate values are present, the expression is
approximate and is evaluated using floating-point arithmetic.
If no approximate values are present, the expression contains
only exact values. If any exact value contains a fractional
part (a value following the decimal point), the expression is
evaluated using DECIMAL exact arithmetic
and has a precision of 65 digits. (The term
“exact” is subject to the limits of what can be
represented in binary. For example, 1.0/3.0
can be approximated in decimal notation as
.333..., but not written as an exact
number, so (1.0/3.0)*3.0 does not evaluate
to exactly 1.0.)
Otherwise, the expression contains only integer values. The
expression is exact and is evaluated using integer arithmetic
and has a precision the same as BIGINT (64
bits).
If a numeric expression contains any strings, they are converted
to double-precision floating-point values and the expression is
approximate.
Inserts into numeric columns are affected by the SQL mode, which
is controlled by the sql_mode system variable.
(See Section 5.2.5, “The Server SQL Mode”.) The following discussion
mentions strict mode (selected by the
STRICT_ALL_TABLES or
STRICT_TRANS_TABLES mode values) and
ERROR_FOR_DIVISION_BY_ZERO. To turn on all
restrictions, you can simply use TRADITIONAL
mode, which includes both strict mode values and
ERROR_FOR_DIVISION_BY_ZERO:
mysql> SET sql_mode='TRADITIONAL';
If a number is inserted into an exact type column
(DECIMAL or integer), it is inserted with its
exact value if it is within the column range.
If the value has too many digits in the fractional part, rounding
occurs and a warning is generated. Rounding is done as described
in Rounding Behavior.
If the value has too many digits in the integer part, it is too
large and is handled as follows:
If strict mode is not enabled, the value is truncated to the
nearest legal value and a warning is generated.
If strict mode is enabled, an overflow error occurs.
Underflow is not detected, so underflow handing is undefined.
By default, division by zero produces a result of
NULL and no warning. With the
ERROR_FOR_DIVISION_BY_ZERO SQL mode enabled,
MySQL handles division by zero differently:
If strict mode is not enabled, a warning occurs.
If strict mode is enabled, inserts and updates involving
division by zero are prohibited, and an error occurs.
In other words, inserts and updates involving expressions that
perform division by zero can be treated as errors, but this
requires ERROR_FOR_DIVISION_BY_ZERO in addition
to strict mode.
Suppose that we have this statement:
INSERT INTO t SET i = 1/0;
This is what happens for combinations of strict and
ERROR_FOR_DIVISION_BY_ZERO modes:
For inserts of strings into numeric columns, conversion from
string to number is handled as follows if the string has
non-numeric contents:
A string that does not begin with a number cannot be used as a
number and produces an error in strict mode, or a warning
otherwise. This includes the empty
string.
A string that begins with a number can be converted, but the
trailing non-numeric portion is truncated. If the truncated
portion contains anything other than spaces, this produces an
error in strict mode, or a warning otherwise.
