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11.2. Numeric Types

MySQL supports all of the standard SQL numeric data types. These types include the exact numeric data types (INTEGER, SMALLINT, DECIMAL, and NUMERIC), as well as the approximate numeric data types (FLOAT, REAL, and DOUBLE PRECISION). The keyword INT is a synonym for INTEGER, and the keyword DEC is a synonym for DECIMAL. For numeric type storage requirements, see Section 11.5, “Data Type Storage Requirements”.

The BIT data type stores bit-field values and is supported for MyISAM, MEMORY, InnoDB, and BDB tables.

As an extension to the SQL standard, MySQL also supports the integer types TINYINT, MEDIUMINT, and BIGINT. The following table shows the required storage and range for each of the integer types.

Type Bytes Minimum Value Maximum Value
    (Signed/Unsigned) (Signed/Unsigned)
TINYINT 1 -128 127
    0 255
SMALLINT 2 -32768 32767
    0 65535
MEDIUMINT 3 -8388608 8388607
    0 16777215
INT 4 -2147483648 2147483647
    0 4294967295
BIGINT 8 -9223372036854775808 9223372036854775807
    0 18446744073709551615

Another extension is supported by MySQL for optionally specifying the display width of an integer value in parentheses following the base keyword for the type (for example, INT(4)). This optional display width specification is used to left-pad the display of values having a width less than the width specified for the column.

The display width does not constrain the range of values that can be stored in the column, nor the number of digits that are displayed for values having a width exceeding that specified for the column.

When used in conjunction with the optional extension attribute ZEROFILL, the default padding of spaces is replaced with zeros. For example, for a column declared as INT(5) ZEROFILL, a value of 4 is retrieved as 00004. Note that if you store larger values than the display width in an integer column, you may experience problems when MySQL generates temporary tables for some complicated joins, because in these cases MySQL assumes that the data fits into the original column width.

All integer types can have an optional (non-standard) attribute UNSIGNED. Unsigned values can be used when you want to allow only non-negative numbers in a column and you need a larger upper numeric range for the column. For example, if an INT column is UNSIGNED, the size of the column's range is the same but its endpoints shift from -2147483648 and 2147483647 up to 0 and 4294967295.

Floating-point and fixed-point types also can be UNSIGNED. As with integer types, this attribute prevents negative values from being stored in the column. However, unlike the integer types, the upper range of column values remains the same.

If you specify ZEROFILL for a numeric column, MySQL automatically adds the UNSIGNED attribute to the column.

For floating-point data types, MySQL uses four bytes for single-precision values and eight bytes for double-precision values.

The FLOAT and DOUBLE data types are used to represent approximate numeric data values. For FLOAT the SQL standard allows an optional specification of the precision (but not the range of the exponent) in bits following the keyword FLOAT in parentheses. MySQL also supports this optional precision specification, but the precision value is used only to determine storage size. A precision from 0 to 23 results in a four-byte single-precision FLOAT column. A precision from 24 to 53 results in an eight-byte double-precision DOUBLE column.

MySQL allows a non-standard syntax: FLOAT(M,D) or REAL(M,D) or DOUBLE PRECISION(M,D). Here, “(M,D)” means than values are displayed with up to M digits in total, of which D digits may be after the decimal point. For example, a column defined as FLOAT(7,4) will look like -999.9999 when displayed. MySQL performs rounding when storing values, so if you insert 999.00009 into a FLOAT(7,4) column, the approximate result is 999.0001.

MySQL treats DOUBLE as a synonym for DOUBLE PRECISION (a non-standard extension). MySQL also treats REAL as a synonym for DOUBLE PRECISION (a non-standard variation), unless the REAL_AS_FLOAT SQL mode is enabled.

For maximum portability, code requiring storage of approximate numeric data values should use FLOAT or DOUBLE PRECISION with no specification of precision or number of digits.

The DECIMAL and NUMERIC data types are used to store exact numeric data values. In MySQL, NUMERIC is implemented as DECIMAL. These types are used to store values for which it is important to preserve exact precision, for example with monetary data.

MySQL 5.1 stores DECIMAL and NUMERIC values in binary format. Previously, they were stored as strings. See Chapter 24, Precision Math.

When declaring a DECIMAL or NUMERIC column, the precision and scale can be (and usually is) specified; for example:

salary DECIMAL(5,2)

In this example, 5 is the precision and 2 is the scale. The precision represents the number of significant digits that are stored for values, and the scale represents the number of digits that can be stored following the decimal point. If the scale is 0, DECIMAL and NUMERIC values contain no decimal point or fractional part.

Standard SQL requires that the salary column be able to store any value with five digits and two decimals. In this case, therefore, the range of values that can be stored in the salary column is from -999.99 to 999.99.

In standard SQL, the syntax DECIMAL(M) is equivalent to DECIMAL(M,0). Similarly, the syntax DECIMAL is equivalent to DECIMAL(M,0), where the implementation is allowed to decide the value of M. MySQL supports both of these variant forms of the DECIMAL and NUMERIC syntax. The default value of M is 10.

The maximum number of digits for DECIMAL or NUMERIC is 65, but the actual range for a given DECIMAL or NUMERIC column can be constrained by the precision or scale for a given column. When such a column is assigned a value with more digits following the decimal point than are allowed by the specified scale, the value is converted to that scale. (The precise behavior is operating system-specific, but generally the effect is truncation to the allowable number of digits.)

The BIT data type is used to store bit-field values. A type of BIT(M) allows for storage of M-bit values. M can range from 1 to 64.

To specify bit values, b'value' notation can be used. value is a binary value written using zeroes and ones. For example, b'111' and b'100000000' represent 7 and 128, respectively. See Section 9.1.5, “Bit-Field Values”.

If you assign a value to a BIT(M) column that is less than M bits long, the value is padded on the left with zeroes. For example, assigning a value of b'101' to a BIT(6) column is, in effect, the same as assigning b'000101'.

When asked to store a value in a numeric column that is outside the data type's allowable range, MySQL's behavior depends on the SQL mode in effect at the time. For example, if no restrictive modes are enabled, MySQL clips the value to the appropriate endpoint of the range and stores the resulting value instead. However, if the mode is set to TRADITIONAL, MySQL rejects a value that is out of range with an error, and the insert fails, in accordance with the SQL standard.

In non-strict mode, when an out-of-range value is assigned to an integer column, MySQL stores the value representing the corresponding endpoint of the column data type range. If you store 256 into a TINYINT or TINYINT UNSIGNED column, MySQL stores 255 or 127, respectively. When a floating-point or fixed-point column is assigned a value that exceeds the range implied by the specified (or default) precision and scale, MySQL stores the value representing the corresponding endpoint of that range.

Conversions that occur due to clipping when MySQL is not operating in strict mode are reported as warnings for ALTER TABLE, LOAD DATA INFILE, UPDATE, and multiple-row INSERT statements. When MySQL is operating in strict mode, these statements fail, and some or all of the values will not be inserted or changed, depending on whether the table is a transactional table and other factors. For details, see Section 5.2.5, “The Server SQL Mode”.

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