mysql float double 類型

        一般在oracle的數字類型都設定為number不帶長度，這個擴充性比較好。

不太需要去關心是整數還是小數，而且長度所佔的空間也是和儲存的數值相關。

mysql中數實值型別占的長度比較固定，對於float的使用心裡沒什麼底。 直接用

數值進行小測一下。

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mysql> show create table tmp_xf_test\G*************************** 1. row ***************************Table: tmp_xf_testCreate Table: CREATE TABLE `tmp_xf_test` (`t1` float(7,4) DEFAULT NULL,`t2` float DEFAULT NULL,`t3` double(7,4) DEFAULT NULL,`t4` double DEFAULT NULL) ENGINE=MyISAM DEFAULT CHARSET=gbk1. mysql> insert into tmp_xf_test values(5.1,5.1,5.1,5.1);|   5.1000 |         5.1 |   5.1000 |                  5.1 | --設定了具體精度的，小數後面會用0補全2. 123456789.123456789| 999.9999 | 1.23457e+08 | 999.9999 |     123456789.123457 | --限制(7,4)的  溢出了3. 123.4567| 123.4567 |     123.457 | 123.4567 |             123.4567 | 4. 123.45678| 123.4568 |     123.457 | 123.4568 |            123.45678 | 5. 1234.456| 999.9999 |     1234.46 | 999.9999 |             1234.456 |  --溢出6. 0.00009|   0.0001 |       9e-05 |   0.0001 |                9e-05 |7. 0.123456789012345678901|   0.1235 |    0.123457 |   0.1235 |    0.123456789012346 |  --16位有效長度8. 12345678901234567890.123456789012345678901| 999.9999 | 1.23457e+19 | 999.9999 | 1.23456789012346e+19 | -- float 7位有效長度，double 16位有效長度9. 12345678901234567890| 999.9999 | 1.23457e+19 | 999.9999 | 1.23456789012346e+19 |

-----------------------具體參考手冊

http://dev.mysql.com/doc/refman/5.1/en/numeric-types.html

        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-precisionFLOAT column. A precision from 24 to 53 results in an eight-byte

double-precision DOUBLE column.

       MySQL allows a nonstandard syntax: FLOAT(M,D) or REAL(M,D) or DOUBLE

PRECISION(M,D). Here, “(M,D)” means than values can be stored 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.

--關於浮點數可以參考http://www.ibm.com/developerworks/cn/java/j-math2.html

mysql> select * from tmp_xf_test;+----------+-------------+----------+----------------------+| t1       | t2          | t3       | t4                   |+----------+-------------+----------+----------------------+|   5.1000 |         5.1 |   5.1000 |                  5.1 || 999.9999 | 1.23457e+08 | 999.9999 |     123456789.123457 || 123.4567 |     123.457 | 123.4567 |             123.4567 || 123.4568 |     123.457 | 123.4568 |            123.45678 || 999.9999 |     1234.46 | 999.9999 |             1234.456 ||   0.0001 |       9e-05 |   0.0001 |                9e-05 ||   0.1235 |    0.123457 |   0.1235 |    0.123456789012346 || 999.9999 | 1.23457e+19 | 999.9999 | 1.23456789012346e+19 || 999.9999 | 1.23457e+19 | 999.9999 | 1.23456789012346e+19 |+----------+-------------+----------+----------------------+9 rows in set (0.00 sec)