SQL Server基于联接2个表的更新和插入

Question

I have 2 tables which are exactly identical: table1 (a, b, c) and table2 (a, b, c)

But every night, t2 gets updated and after that update, I want to join these two tables and if the the row exists in t1 , then update t1.b , but if not then insert the row from t2 into t1 .

I know this works because it is select

SELECT 
    (CASE
        WHEN t1.a IS NOT NULL 
            THEN t1.b + t2.b
        ELSE t2.b
    END) AS 'total new amount'
FROM
    table1 t1
RIGHT JOIN 
    table2 t2 ON t2.a = t1.a AND t2.c = t1.c

But how can I do the update and insert?

UPDATE:

so I figured I have to use MERGE command

MERGE table1 as t1 
USING table2 as t2 
ON (t1.a = t2.a AND t1.c = t2.c ) 
when MATCHED T
HEN UPDATE SET t1.b = t1.b + t2.b 
when NOT MATCHED 
THEN insert (a,b,c) VALUES (t2.a,t2.b,t2.c);

but now i am getting this error:

The MERGE statement attempted to UPDATE or DELETE the same row more than once. This happens when a target row matches more than one source row. A MERGE statement cannot UPDATE/DELETE the same row of the target table multiple times. Refine the ON clause to ensure a target row matches at most one source row, or use the GROUP BY clause to group the source rows.

what I am doing wrong here?

Thanks in advance

Answer 1

But every night, t2 gets updated and after that update, I want to join these two tables and if the the row exists in t1, then update t1.b, but if not then insert the row from t2 into t1.

Does anything happen to t1?

If nothing changes in t1 , the simplest way to sync the tables is to just truncate and reload

Depending on how many records in the table and the rate of change, this might be faster or slower than doing an upsert.

TRUNCATE TABLE T1;

INSERT INTO T1 (Col1,Col2,Col3)
SELECT Col1,Col2,Col3 FROM T2;

Depending on how frequently T1 is used, there are ways to make this transparent to the end user. (ie they'll never see an empty table)

Some methods are:

• Use snapshot isolation and a transaction
• Use a staging table and SWITCH PARTITION https://littlekendra.com/2017/01/19/why-you-should-switch-in-staging-tables-instead-of-renaming/
• Use a staging table and sp_rename

MERGE is certainly one way to do an upsert but seperate INSERT and UPDATE statements are often easier to debug and perform better.