sql-server - 兼容模式允许新命令？

Question

我有一个在 2012 兼容模式下运行数据库的 SQL Server 2017 服务器。但是，当我尝试使用新到 2017 T-SQL 命令string_agg()时，它可以工作。我预计会出现语法错误，因为string_agg()它对 2012 模式无效。

这是我正在使用的 SQL：

SELECT
    Compatibility_Level,
    CASE Compatibility_Level
       WHEN 65 THEN 'SQL Server 6.5'
       WHEN 70 THEN 'SQL Server 7.0'
       WHEN 80 THEN 'SQL Server 2000'
       WHEN 90 THEN 'SQL Server 2005'
       WHEN 100 THEN 'SQL Server 2008/R2'
       WHEN 110 THEN 'SQL Server 2012'
       WHEN 120 THEN 'SQL Server 2014'
       WHEN 130 THEN 'SQL Server 2016'
       WHEN 140 THEN 'SQL Server 2017 <<<'
       ELSE 'new unknown - ' + CONVERT(VARCHAR(10), Compatibility_Level)
    END AS VersionName
FROM 
    sys.databases
WHERE
    Name = DB_NAME()

DECLARE @x TABLE (col1 INT, col2 VARCHAR(5))

INSERT INTO @x 
VALUES (1, 1), (1, 2), (1, 3), (2, 1), (2, 2)

SELECT * FROM @x

SELECT col1, string_agg(col2,', ') 
FROM @x 
GROUP BY col1

这是输出：

Compatibility_Level VersionName
------------------- ------------------------
110                 SQL Server 2012

(1 row affected)

(5 rows affected)

col1        col2
----------- -----
1           1
1           2
1           3
2           1
2           2

(5 rows affected)

col1        
----------- ------------------------------------
1           1, 2, 3
2           1, 2

(2 rows affected)

标签： sql-servertsqlsql-server-2012sql-server-2017

Answer 1

The purpose of Compatibility Mode is to ensure that statements which were valid in previous versions of SQL Server behave in the same way in newer versions of SQL server.

This includes statements which were previously valid but have become invalid in the new version, and statements which have a different meaning or behave differently between versions.

Statements which were invalid in previous versions are not included in this goal.

Why? Consider upgrading. You first want to convert everything to the new syntax and functions, then when you are satisfied everything is ready, you can raise the compatibility level. If you can't allow things which are valid in the new compatibility level (but invalid or meaningless before) then it becomes difficult to migrate to the higher level.