When an operator is used with operands of different types, type conversion occurs to make the operands compatible. Some conversions occur implicitly. For example, MySQL automatically converts numbers to strings as necessary, and vice versa.
SELECT 1+'1';-> 2 mysql>
SELECT CONCAT(2,' test');-> '2 test'
It is also possible to convert a number to a string explicitly
Conversion occurs implicitly with the
CONCAT() function because it
expects string arguments. (
is preferable, but is unavailable before MySQL 4.0.2.)
SELECT 38.8, CAST(38.8 AS CHAR);-> 38.8, '38.8' mysql>
SELECT 38.8, CONCAT(38.8);-> 38.8, '38.8'
See later in this section for information about the character set of implicit number-to-string conversions.
The following rules describe how conversion occurs for comparison operations:
If one or both arguments are
result of the comparison is
equality comparison operator. For
NULL, the result is true. No conversion is needed.
If both arguments in a comparison operation are strings, they are compared as strings.
If both arguments are integers, they are compared as integers.
Hexadecimal values are treated as binary strings if not compared to a number.
If one of the arguments is a
DATETIME column and the other
argument is a constant, the constant is converted to a
timestamp before the comparison is performed. This is done to
be more ODBC-friendly. Note that this is not done for the
IN()! To be safe,
always use complete datetime, date, or time strings when doing
comparisons. For example, to achieve best results when using
BETWEEN with date or time values,
CAST() to explicitly
convert the values to the desired data type.
In all other cases, the arguments are compared as floating-point (real) numbers.
For information about conversion of values from one temporal type to another, see Section 10.3.5, “Conversion Between Date and Time Types”.
The following examples illustrate conversion of strings to numbers for comparison operations:
SELECT 1 > '6x';-> 0 mysql>
SELECT 7 > '6x';-> 1 mysql>
SELECT 0 > 'x6';-> 0 mysql>
SELECT 0 = 'x6';-> 1
For comparisons of a string column with a number, MySQL cannot use
an index on the column to look up the value quickly. If
str_col is an indexed string column,
the index cannot be used when performing the lookup in the
SELECT * FROM
The reason for this is that there are many different strings that
may convert to the value
1, such as
' 1', or
Comparisons that use floating-point numbers (or values that are converted to floating-point numbers) are approximate because such numbers are inexact. This might lead to results that appear inconsistent:
SELECT '18015376320243458' = 18015376320243458;-> 1 mysql>
SELECT '18015376320243459' = 18015376320243459;-> 0
Such results can occur because the values are converted to floating-point numbers, which have only 53 bits of precision and are subject to rounding:
SELECT '18015376320243459'+0.0;-> 1.8015376320243e+16
Furthermore, the conversion from string to floating-point and from integer to floating-point do not necessarily occur the same way. The integer may be converted to floating-point by the CPU, whereas the string is converted digit by digit in an operation that involves floating-point multiplications.
The results shown will vary on different systems, and can be
affected by factors such as computer architecture or the compiler
version or optimization level. One way to avoid such problems is
CAST() so that a value will
not be converted implicitly to a float-point number:
SELECT CAST('18015376320243459' AS UNSIGNED) = 18015376320243459;-> 1
For more information about floating-point comparisons, see Section B.5.5.8, “Problems with Floating-Point Values”.
Implicit conversion of a numeric or temporal value to a string
produces a binary string (a
BLOB value). Such implicit
conversions to string typically occur for functions that are
passed numeric or temporal values when string values are more
usual, and thus can have effects beyond the type of the converted
value. Consider the expression
'abc'). The numeric argument
converted to the binary string
'1' and the
concatenation of that value with the nonbinary string
'abc' produces the binary string