与MySQL中不同数据库中的另一个表相比，如何从一个表中获取缺少的行？

Question

I have two tables that have more than 200k rows, they are supposed to be identical except that one has 3 rows more than the other. I am trying to figure out which lines are they. And each table is in a different database. How can I do so in MySQL?

I tried this:

SELECT t1.* 
FROM db1.tb1 t1 
LEFT JOIN db2.tb2 ON tb1.col_13 = tb2.col_13 
WHERE tb2.col_13 IS NULL;

but it is taking FOREVER.

EDIT

Since col_13 is all duplicates, this wont work. The problem is I cant find a commen primary key between the tables, the primary key between them is datetime, which is almost identical, but because of different scripts used to insert the data into each table, some of the rows have different seconds because of rounding, for example: "2015-09-01 00:00:11" and "2015-09-01 00:00:12" are the same rows but because of rounding they have different seconds.

Answer 1

Easton's comments are correct. You are executing the query properly. Your issue isn't with joining two databases but with your query performance. To solve that problem, more details about your table structure will be required. His suggestions are good starting points though, namely make sure and do the join on a column which is unique and indexed in both databases. That will allow the query to execute as quickly as possible.

If you can't easily fix the problem yourself, you may be best off asking a new question and this time focusing on the performance with more details about the table structure.

Answer 2

This should take just a second or two since your query looks bang-on, so you may need to add indexes to col_13. If this only runs once or twice than you may not need that, but if it runs often, I would add indexes.