SQL SERVER UPDATE的赋值次序

1) 先变量再字段

SET NOCOUNT ON;

DECLARE @i INT, @j INT
DECLARE @Table TABLE
(
 Id1 INT,
 Id2 INT
);

INSERT @Table(Id1, Id2)
SELECT 1, 10
UNION ALL
SELECT 2, 20
UNION ALL
SELECT 3, 30;

SELECT @i = 1, @j = 0
UPDATE @Table SET Id1 = @i, Id2 = Id1, @i = @i + 1
SELECT * FROM @Table

SELECT @i = 1, @j = 0
UPDATE @Table SET Id1 = @i, Id2 = @j, @j = @i + 10, @i = @i + 1
SELECT * FROM @Table

SET NOCOUNT OFF;
结果:
Id1         Id2        
----------- -----------
2           1
3           2
4           3

Id1         Id2        
----------- -----------
2           11
3           12
4           13

2) 变量之间, 从左到右

SET NOCOUNT ON;

DECLARE @i INT, @j INT
DECLARE @Table TABLE
(
    Id1 INT,
    Id2 INT
);

INSERT @Table(Id1, Id2)
SELECT 1, 10;

SELECT @i = 1, @j = 0
UPDATE @Table SET @j = @i, @i = @i + 1
PRINT '@i = ' + CAST(@i AS VARCHAR) + ', @j = ' + CAST(@j AS VARCHAR)

SELECT @i = 1, @j = 0
UPDATE @Table SET @i = @i + 1, @j = @i
PRINT '@i = ' + CAST(@i AS VARCHAR) + ', @j = ' + CAST(@j AS VARCHAR)

SELECT @i = 1, @j = 0
UPDATE @Table SET @i = @j + 1, @j = @i
PRINT '@i = ' + CAST(@i AS VARCHAR) + ', @j = ' + CAST(@j AS VARCHAR)

SELECT @i = 1, @j = 0
UPDATE @Table SET @i = @j, @j = @i
PRINT '@i = ' + CAST(@i AS VARCHAR) + ', @j = ' + CAST(@j AS VARCHAR)

SET NOCOUNT OFF;
结果:
@i = 2, @j = 1
@i = 2, @j = 2
@i = 1, @j = 1
@i = 0, @j = 0

3) 字段之间, 并行执行

SET NOCOUNT ON;

DECLARE @Table TABLE
(
 Id1 INT, Id2 INT
);

INSERT @Table(Id1, Id2)
SELECT 1, 10
UNION ALL
SELECT 2, 20
UNION ALL
SELECT 3, 30;

UPDATE @Table SET Id1 = Id2, Id2 = Id1
SELECT * FROM @Table

SET NOCOUNT OFF;
结果:
Id1         Id2        
----------- -----------
10          1
20          2
30          3