sql - Float 在数据库 Sql Server 中存储为 Real

Question

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？

Answer 1

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)。