With precision math, exactvalue 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 permissible.)
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 floatingpoint 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 doubleprecision floatingpoint 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 5.1.7,
"Server SQL Modes".) 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 Section 12.19.4, "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 handling 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.
sql_mode
Value

Result 

'' (Default) 
No warning, no error; i is set to NULL .

strict  No warning, no error; i is set to NULL .

ERROR_FOR_DIVISION_BY_ZERO 
Warning, no error; i is set toNULL . 
strict,ERROR_FOR_DIVISION_BY_ZERO 
Error condition; no row is inserted. 
For inserts of strings into numeric columns, conversion from string to number is handled as follows if the string has nonnumeric 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 nonnumeric portion is truncated. If the truncated portion contains anything other than spaces, this produces an error in strict mode, or a warning otherwise.