SQL Server中Upsert的三种方式

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本文介绍了SQL Server中Upsert的三种常见写法以及他们的性能比较。

SQL Server并不支持原生的Upsert语句，通常使用组合语句实现upsert功能。

假设有表table_A，各字段如下所示：

int型Id为主键。

方法1：先查询，根据查询结果判断使用insert或者update

IF EXISTS (SELECT 1 FROM table_A WHERE Id = @Id)
    BEGIN
        UPDATE dbo.table_A
        SET Value = @Value
        WHERE Id = @Id;
    END
ELSE
    BEGIN
        INSERT INTO dbo.table_A (Id, Value)
        VALUES(@Id, @Value)
    END

方法2：先更新，根据更新结果影响的条目数判断是否需要插入

UPDATE dbo.table_A
SET Value = @Value
WHERE Id = @Id;
    
IF(@@ROWCOUNT = 0)
BEGIN
    INSERT INTO dbo.table_A (Id, Value)
    VALUES(@Id, @Value)
END

方法3：使用MERGE语句，将待upsert的数据作为source，merge到目标表 

MERGE INTO table_A as T
USING (SELECT @Id AS id, @Value AS value ) AS S
ON T.Id = S.id
WHEN MATCHED THEN
    UPDATE SET T.Value = S.value
WHEN NOT MATCHED THEN
    INSERT(Id, Value) VALUES(S.id, S.value);

性能比较

在50万行数据项中随机upsert10万次

场景一：upsert数据项100%命中update

场景二：upsert数据项100%命中insert

场景三：upsert数据项Id为随机数，~50%insert，~50%update

从图中可以看出实验数据存在部分偏差，大体上这三种方法在性能上的差别非常小。对于绝大多数upsert并非关键路径的程序，方法2在可读性和执行性能上综合来讲是较优的方案。

在对性能苛求的场景，可以选用MERGE语句，以下是MERGE语句的优点：”

1. Faster performance. The Engine needs to parse, compile, and execute only one query instead of three (and no temporary variable to hold the key).
2. Neater and simpler T-SQL code (after you get proficient in MERGE).
3. No need for explicit BEGIN TRANSACTION/COMMIT. MERGE is a single statement and is executed in one implicit transaction.
4. Greater functionality. MERGE can delete rows that are not matched by source (SRC table above). For example, we can delete row 1 from A_Table because its Data column does not match Search_Col in the SRC table. There is also a way to return inserted/deleted values using the OUTPUT clause.“

引用：http://www.sergeyv.com/blog/archive/2010/09/10/sql-server-upsert-equivalent.aspx

SQL Server中Upsert的三种方式

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原文地址：http://www.cnblogs.com/zhenfengren/p/5618511.html