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12.1.2. Type Conversion in Expression Evaluation

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.

mysql> SELECT 1+'1';
        -> 2
mysql> SELECT CONCAT(2,' test');
        -> '2 test'

It is also possible to perform explicit conversions. If you want to convert a number to a string explicitly, use the CAST() or CONCAT() function (CAST() is preferable):

mysql> SELECT 38.8, CAST(38.8 AS CHAR);
        -> 38.8, '38.8'
mysql> SELECT 38.8, CONCAT(38.8);
        -> 38.8, '38.8'

The following rules describe how conversion occurs for comparison operations:

  • If one or both arguments are NULL, the result of the comparison is NULL, except for the NULL-safe <=> equality comparison operator. For NULL <=> NULL, the result is true.

  • 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 TIMESTAMP or 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 arguments to IN()! To be safe, always use complete datetime/date/time strings when doing comparisons.

  • In all other cases, the arguments are compared as floating-point (real) numbers.

The following examples illustrate conversion of strings to numbers for comparison operations:

mysql> SELECT 1 > '6x';
        -> 0
mysql> SELECT 7 > '6x';
        -> 1
mysql> SELECT 0 > 'x6';
        -> 0
mysql> SELECT 0 = 'x6';
        -> 1

Note that when you are comparing 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 following statement:

SELECT * FROM tbl_name WHERE str_col=1;

The reason for this is that there are many different strings that may convert to the value 1: '1', ' 1', '1a', …


 
 
  Published under the terms of the GNU General Public License Design by Interspire