復制代碼 代碼如下:

SET @SQL = 'SELECT * FROM Comment with(nolock) WHERE 1=1
    And (@ProjectIds Is Null or ProjectId = @ProjectIds)
    And (@Scores is null or Score =@Scores)'

復制代碼 代碼如下:

CREATE TABLE aaa(id int IDENTITY, NAME VARCHAR(12), age INT)
go
CREATE INDEX idx_age ON aaa (age)
GO

2、插入1萬條測試數據：

復制代碼 代碼如下:

DECLARE @i INT;
SET @i=0;
WHILE @i<10000
BEGIN
  INSERT INTO aaa (name, age)VALUES(CAST(@i AS VARCHAR), @i)
  SET @i=@i+1;
END
GO

4、開始測試，用下面的SQL進行測試：

復制代碼 代碼如下:

DECLARE @i INT;
SET @i=100
SELECT * FROM aaa WHERE (@i IS NULL OR age = @i)
SELECT * FROM aaa WHERE (age = @i OR @i IS NULL)
SELECT * FROM aaa WHERE age=isnull(@i, age)
SELECT * FROM aaa WHERE age = @i

可以看到，即使@i有值，不管@i IS NULL是放在前面還是放在后面，都無法用到age的索引，另外age=ISNULL(@i,age)也用不上索引

最終結論，SQL Server跟ORACLE一樣，如果條件里加了 變量 IS NULL，都會導致全表掃描。

建議SQL改成：

復制代碼 代碼如下:

DECLARE @i INT;
SET @i=100

DECLARE @sql NVARCHAR(MAX)
SET @sql = 'SELECT * FROM aaa'
IF @i IS NOT NULL
    SET @sql = @sql + ' WHERE age = @i'
EXEC sp_executesql @sql, N'@i int', @i

復制代碼 代碼如下:

DECLARE @i INT;
SET @i=100
IF @i IS NOT NULL
    SELECT * FROM aaa WHERE age = @i
ELSE
    SELECT * FROM aaa

但是，如果條件多了，SQL數目也變得更多，所以建議用EXEC的方案