MySQL - 跨越两个表的最佳实践

Question

I have two tables. Each one is populated with about 50K records. Both have one common field. I want the records of both tables where the common field is matching. So I use the following SQL code. The problem is that the query is running now for already 20 minutes and nothing happening. It seems strange to me that it is taking so much time. I wonder if there is a better way to achieve my target. Thank you in advance for your replies.

My SQL code:

Select * from tableOne T1, tableTwo T2 
Where T1.name = T2.name

By the way I am open to php solutions (loops or whatever if it is better...)

Answer 1

Two things:

1. You need to make sure there is an index on the name column in both tables. Otherwise the query will have to essentially scan every row of each table looking for matches.

2. Ideally, the index should have "included columns" that you want to select in the query. Otherwise the query won't be able to fully use the index, but will have to go back to the table and pick up the other columns manually for each row. This means you should consider selecting only the columns you need (rather than *) in the query, and adding those columns as included columns to the index.

Answer 2

Try this:

select *
from tableOne t1
inner join tableTwo t2 on t1.name = t2.name

Althought I doubt that would make a difference, trying won't hurt. It's not recommended to use from tableOne t1, tableTwo t2 , because you are implicitly making a cartesian product!

Answer 3

I believe it's better to use Join

Something like this:

SELECT *
FROM tableOne
INNER JOIN TableTwo
ON tableOne.name=TableTwo.name

Answer 4

应该是一个内部联接，以从两个表中获取名称连接的行。

SELECT t1.*, t2.* FROM table1 t1 INNER JOIN table2 t2 ON t1.name = t2.name

Answer 5

MySQL Inner Joins - what you have happening there - are very efficient. 50K records is not very much at all, and twenty minutes is ridiculously long even for a pair of tables with a couple million entries. Try to restart the query, or just restart the whole web server. The query itself if fine (although it's good practice to specify exact which columns from each table you want, even if you want every column except one). Adding an index if you don't have one is also a good idea.

Answer 6

What is your tables' storage engine? If it is InnoDB, you would need to set a foreign key between these two tables -- that will definitely solve your performance problem.