原文鏈接： Mysql中float列的等值查詢
注：float 類型的數據在比較相等時，無論是在c,c++,java,php 等語言比較時，都需要特別注意。
mysql中float類型不定義精度時，對float列的檢索可能不正確。即如果有記錄存在 field_float_a 值爲1.234, 查詢select * from table_test where field_float_a = 1.234. 可能查詢不到數據。
1.創建表t3,col1_float定義爲float類型不指定精度
mysql> CREATE TABLE `t3` (
`col1_float` float DEFAULT NULL,
`col2_double` double DEFAULT NULL,
`col3_decimal` decimal DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;
Query OK, 0 rows affected
2.插入數據
mysql中float類型不指定精度時，會按照實際的精度顯示
mysql> insert into t3 values(100.123,200.123,300.123);
Query OK, 1 row affected
mysql> select * from t3;
+------------+-------------+--------------+
| col1_float | col2_double | col3_decimal |
+------------+-------------+--------------+
| 100.123 | 200.123 | 300 |
+------------+-------------+--------------+
1 row in set
3.對float類型不指定精度時，默認等值查詢結果爲空.
如t3表中存在col1_float爲100.123的記錄，但由於定義col1_float的類型時沒有指定float的精度，在對float列進行等值查詢的時候檢索不到記錄。
mysql> select * from t3 where col1_float=100.123;
Empty set
mysql> show warnings;
Empty set
4.對double類型，則沒有這種情況
mysql> select * from t3 where col2_double=200.123;
+------------+-------------+--------------+
| col1_float | col2_double | col3_decimal |
+------------+-------------+--------------+
| 100.123 | 200.123 | 300 |
+------------+-------------+--------------+
1 row in set
4.對decimal類型，如果不指定精度, 會對小數位進行截斷
mysql> select * from t3 where col3_decimal=300;
+------------+-------------+--------------+
| col1_float | col2_double | col3_decimal |
+------------+-------------+--------------+
| 100.123 | 200.123 | 300 |
+------------+-------------+--------------+
1 row in set
Mysql中針對不定義精度的float類型進行等值查詢的解決方法：
a).使用like
mysql> select * from t3 where col1_float like 100.123;
+------------+-------------+--------------+
| col1_float | col2_double | col3_decimal |
+------------+-------------+--------------+
| 100.123 | 200.123 | 300 |
+------------+-------------+--------------+
1 row in set
b).使用format進行轉換
mysql> select * from t3 where format(col1_float ,3) = format(100.123 ,3);
+------------+-------------+--------------+
| col1_float | col2_double | col3_decimal |
+------------+-------------+--------------+
| 100.123 | 200.123 | 300 |
+------------+-------------+--------------+
1 row in set
c). 修改數據類型，指定精度和標度。
mysql> alter table t3 modify col1_float float(6,3);
Query OK, 0 rows affected
Records: 0 Duplicates: 0 Warnings: 0
mysql> select * from t3 where col1_float=100.123;
+------------+-------------+--------------+
| col1_float | col2_double | col3_decimal |
+------------+-------------+--------------+
| 100.123 | 200.123 | 300 |
+------------+-------------+--------------+
1 row in set
---------------------
作者：lwei_998
來源：CSDN
原文：https://blog.csdn.net/lwei_998/article/details/40979351
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