MySQL 更新加入未按预期工作

Question

I'm using MySQL 5.1.41 on ubuntu 12.10 and MySQL Workbench.

I have 2 product tables, t1 and t2. t1 is the live data and t2 is a imported data ready to be updated into t1 to update all the new product prices. So I run:

SELECT * FROM t1
JOIN t2 ON t1.id = t2.id
WHERE t1.price != t2.price;

This returns 1201 records where the price is different and needs to be updated. So I run:

UPDATE t1 JOIN t2 ON t1.id = t2.id
SET t1.price = t2.price
WHERE t1.price != t2.price;

This completes without error and reports 1143 row(s) affected, Rows matched: 1143 Changed: 1143 Warnings: 0

So already something here is not right. 1201 records were different in the select query, but only 1143 changed using the same join and criteria?

Running the initial select query I'd expect to see 58 records that still had different prices. But when running it I get the same 1201 as I did initially. It's as if the updates are not being committed.

Any ideas?

Answer 1

The number ( 1201 ) that your SELECT shows is not records of t1 but rows from the JOIN of two tables. If the two id are not both UNIQUE or PRIMARY KEY s then this is expected. Some rows of t1 match multiple rows from t2 . But when the UPDATE is done they are only updated once (this is a MySQL "feature" or "bug" of UPDATE that checks WHERE conditions sequentially during an update statement.

Try this to see how many rows (of t1 ) should be updated:

SELECT * FROM t1
WHERE EXISTS
      ( SELECT *
        FROM t2
        WHERE t1.id = t2.id
          AND t1.price != t2.price
      );