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
----------- -----------
21
32
43

Id1 Id2
----------- -----------
211
312
413

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