为什么是Float存储为 Real在sys.columns或 Information_schema.columns什么时候precision <= 24 .

CREATE TABLE dummy 
  ( 
     a FLOAT(24), 
     b FLOAT(25) 
  )  

检查数据类型

SELECT TABLE_NAME, 
       COLUMN_NAME, 
       DATA_TYPE, 
       NUMERIC_PRECISION 
FROM   INFORMATION_SCHEMA.COLUMNS 
WHERE  TABLE_NAME = 'dummy'  

结果:

+------------+-------------+-----------+-------------------+ 
| TABLE_NAME | COLUMN_NAME | DATA_TYPE | NUMERIC_PRECISION | 
+------------+-------------+-----------+-------------------+ 
| dummy      | a           | real      |                24 | 
| dummy      | b           | float     |                53 | 
+------------+-------------+-----------+-------------------+ 

那为什么是float存储为 real当precision小于或等于 24 .这在某处记录了吗？

请您参考如下方法:

来自MSDN article其中讨论了 T-SQL 中 float 和 real 之间的区别:

The ISO synonym for real is float(24).

float [ (n) ] 

Where n is the number of bits that are used to store the mantissa of the float number in scientific notation and, therefore, dictates the precision and storage size. If n is specified, it must be a value between 1 and 53. The default value of n is 53.

n value | Precision | Storage size 
1-24    | 7 digits  | 4 bytes 
24-53   | 15 digits | 8 bytes 

SQL Server treats n as one of two possible values. If 1<=n<=24, n is treated as 24. If 25<=n<=53, n is treated as 53.

至于为什么SQL Server把它标为real，我觉得只是一个同义词而已。然而，在引擎盖下它仍然是一个 float(24)。