MySQL浮點型和定點型可以用類型名稱後加（M，D）來表示，M表示該值的總共長度，D表示小數點後面的長度，M和D又稱為精度和標度，如float(7,4)的可顯示為-999.9999，MySQL保存值時進行四捨五入，如果插入999.00009，則結果為999.0001。FLOAT和DOUBLE在不指定精度時，默認會按照實際的精度來顯示，而DECIMAL在不指定精度時，默認整數為10，小數為0。
創建下表：
mysql> CREATE TABLE t2(id1 FLOAT(5,2) DEFAULT NULL,id2 DOUBLE(5,2) DEFAULT NULL,id3 DECIMAL(5,2) DEFAULT NULL);
mysql> DESC t2;
+-------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id1 | float(5,2) | YES | | NULL | |
| id2 | double(5,2) | YES | | NULL | |
| id3 | decimal(5,2) | YES | | NULL | |
+-------+--------------+------+-----+---------+-------+
往id1,id2,id3這三個字段中插入數據1.23：
mysql> INSERT INTO t2(id1,id2,id3) VALUES(1.23,1.23,1.23);
mysql> SELECT * FROM t2;
+------+------+------+
| id1 | id2 | id3 |
+------+------+------+
| 1.23 | 1.23 | 1.23 |
+------+------+------+
數據都正確插入，再向id1插入1.234，id2插入1.234，id3仍然插入1.23：
mysql> INSERT INTO t2(id1,id2,id3) VALUES(1.234,1.234,1.23);
mysql> SELECT * FROM t2;
+------+------+------+
| id1 | id2 | id3 |
+------+------+------+
| 1.23 | 1.23 | 1.23 |
| 1.23 | 1.23 | 1.23 |
+------+------+------+
數據全部正確插入，但是id1和id2由於標度的限制，捨去了最後一位。
同時向id1,id2,id3中插入數據1.234：
mysql> INSERT INTO t2(id1,id2,id3) VALUES(1.234,1.234,1.234);
Query OK, 1 row affected, 1 warning (0.02 sec)
mysql> SELECT * FROM t2;
+------+------+------+
| id1 | id2 | id3 |
+------+------+------+
| 1.23 | 1.23 | 1.23 |
| 1.23 | 1.23 | 1.23 |
| 1.23 | 1.23 | 1.23 |
+------+------+------+
3 rows in set (0.00 sec)
數據也插入成功，但是有一個錯誤提示，
mysql> SHOW warnings;
+-------+------+------------------------------------------+
| Level | Code | Message |
+-------+------+------------------------------------------+
| Note | 1265 | Data truncated for column 'id3' at row 1 |
+-------+------+------------------------------------------+
1 row in set (0.00 sec)
將id1,id2,id3的精度和標度都去掉，再插入數據1.234：
mysql> ALTER TABLE t2 MODIFY id1 FLOAT;
Query OK, 0 rows affected (0.14 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE t2 MODIFY id2 DOUBLE;
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE t2 MODIFY id3 DECIMAL;
Query OK, 4 rows affected, 4 warnings (0.06 sec)
Records: 4 Duplicates: 0 Warnings: 4
mysql> DESC t2;
+-------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------+------+-----+---------+-------+
| id1 | float | YES | | NULL | |
| id2 | double | YES | | NULL | |
| id3 | decimal(10,0) | YES | | NULL | |
+-------+---------------+------+-----+---------+-------+
mysql> INSERT INTO t2(id1,id2,id3) VALUES(1.234,1.234,1.234);
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> SHOW WARNINGS;
+-------+------+------------------------------------------+
| Level | Code | Message |
+-------+------+------------------------------------------+
| Note | 1265 | Data truncated for column 'id3' at row 1 |
+-------+------+------------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT * FROM t2;
+-------+-------+------+
| id1 | id2 | id3 |
+-------+-------+------+
| 1.234 | 1.234 | 1 |
+-------+-------+------+
1 row in set (0.00 sec)
id1和id2的數據正確插入，而id3被截斷。
浮點數如果不寫精度和標度，則會按照實際顯示，如果有精度和標度，則會將數據四捨五入後插入，系統不報錯，定點數如果不設置精度和標度，剛按照默認的（10,0）進行操作，如果數據超過了精度和標度值，則會報錯。
