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This section provides some examples that show precision math query results in MySQL. These examples demonstrate the principles described inSection 14.25.3, “Expression Handling”, andSection 14.25.4, “Rounding Behavior”.

Example 1. Numbers are used with their exact value as given when possible:

mysql> SELECT (.1 + .2) = .3;+----------------+| (.1 + .2) = .3 |+----------------+|              1 |+----------------+

For floating-point values, results are inexact:

mysql> SELECT (.1E0 + .2E0) = .3E0;+----------------------+| (.1E0 + .2E0) = .3E0 |+----------------------+|                    0 |+----------------------+

Another way to see the difference in exact and approximate value handling is to add a small number to a sum many times. Consider the following stored procedure, which adds.0001 to a variable 1,000 times.

CREATE PROCEDURE p ()BEGIN  DECLARE i INT DEFAULT 0;  DECLARE d DECIMAL(10,4) DEFAULT 0;  DECLARE f FLOAT DEFAULT 0;  WHILE i < 10000 DO    SET d = d + .0001;    SET f = f + .0001E0;    SET i = i + 1;  END WHILE;  SELECT d, f;END;

The sum for bothd andf logically should be 1, but that is true only for the decimal calculation. The floating-point calculation introduces small errors:

+--------+------------------+| d      | f                |+--------+------------------+| 1.0000 | 0.99999999999991 |+--------+------------------+

Example 2. Multiplication is performed with the scale required by standard SQL. That is, for two numbersX1 andX2 that have scaleS1 andS2, the scale of the result isS1 +S2:

mysql> SELECT .01 * .01;+-----------+| .01 * .01 |+-----------+| 0.0001    |+-----------+

Example 3. Rounding behavior for exact-value numbers is well-defined:

Rounding behavior (for example, with theROUND() function) is independent of the implementation of the underlying C library, which means that results are consistent from platform to platform.

Example 4. In strict mode, inserting a value that is out of range for a column causes an error, rather than truncation to a legal value.

When MySQL is not running in strict mode, truncation to a legal value occurs:

mysql> SET sql_mode='';Query OK, 0 rows affected (0.00 sec)mysql> CREATE TABLE t (i TINYINT);Query OK, 0 rows affected (0.01 sec)mysql> INSERT INTO t SET i = 128;Query OK, 1 row affected, 1 warning (0.00 sec)mysql> SELECT i FROM t;+------+| i    |+------+|  127 |+------+1 row in set (0.00 sec)

However, an error occurs if strict mode is in effect:

mysql> SET sql_mode='STRICT_ALL_TABLES';Query OK, 0 rows affected (0.00 sec)mysql> CREATE TABLE t (i TINYINT);Query OK, 0 rows affected (0.00 sec)mysql> INSERT INTO t SET i = 128;ERROR 1264 (22003): Out of range value adjusted for column 'i' at row 1mysql> SELECT i FROM t;Empty set (0.00 sec)

Example 5: In strict mode and withERROR_FOR_DIVISION_BY_ZERO set, division by zero causes an error, not a result ofNULL.

In nonstrict mode, division by zero has a result ofNULL:

mysql> SET sql_mode='';Query OK, 0 rows affected (0.01 sec)mysql> CREATE TABLE t (i TINYINT);Query OK, 0 rows affected (0.00 sec)mysql> INSERT INTO t SET i = 1 / 0;Query OK, 1 row affected (0.00 sec)mysql> SELECT i FROM t;+------+| i    |+------+| NULL |+------+1 row in set (0.03 sec)

However, division by zero is an error if the proper SQL modes are in effect:

mysql> SET sql_mode='STRICT_ALL_TABLES,ERROR_FOR_DIVISION_BY_ZERO';Query OK, 0 rows affected (0.00 sec)mysql> CREATE TABLE t (i TINYINT);Query OK, 0 rows affected (0.00 sec)mysql> INSERT INTO t SET i = 1 / 0;ERROR 1365 (22012): Division by 0mysql> SELECT i FROM t;Empty set (0.01 sec)

Example 6. Exact-value literals are evaluated as exact values.

Approximate-value literals are evaluated using floating point, but exact-value literals are handled asDECIMAL:

mysql> CREATE TABLE t SELECT 2.5 AS a, 25E-1 AS b;Query OK, 1 row affected (0.01 sec)Records: 1  Duplicates: 0  Warnings: 0mysql> DESCRIBE t;+-------+-----------------------+------+-----+---------+-------+| Field | Type                  | Null | Key | Default | Extra |+-------+-----------------------+------+-----+---------+-------+| a     | decimal(2,1) unsigned | NO   |     | 0.0     |       || b     | double                | NO   |     | 0       |       |+-------+-----------------------+------+-----+---------+-------+2 rows in set (0.01 sec)

Example 7. If the argument to an aggregate function is an exact numeric type, the result is also an exact numeric type, with a scale at least that of the argument.

Consider these statements:

mysql> CREATE TABLE t (i INT, d DECIMAL, f FLOAT);mysql> INSERT INTO t VALUES(1,1,1);mysql> CREATE TABLE y SELECT AVG(i), AVG(d), AVG(f) FROM t;

The result is a double only for the floating-point argument. For exact type arguments, the result is also an exact type:

mysql> DESCRIBE y;+--------+---------------+------+-----+---------+-------+| Field  | Type          | Null | Key | Default | Extra |+--------+---------------+------+-----+---------+-------+| AVG(i) | decimal(14,4) | YES  |     | NULL    |       || AVG(d) | decimal(14,4) | YES  |     | NULL    |       || AVG(f) | double        | YES  |     | NULL    |       |+--------+---------------+------+-----+---------+-------+

The result is a double only for the floating-point argument. For exact type arguments, the result is also an exact type.