MySQL合并两个优先级的表

Question

In a MySQL database, I have 2 tables with the following sample data. The table key is variable_id.

table_1

**variable_id** **release_id**  
     3               10
     5               10

table_2

**variable_id** **release_id**  
     1               01
     2               15
     3               15
     4               20
     5               25

I want to be able to merge the two tables, with the rows in table_1 having precedence over the rows in table_2.

The result set should be:

**variable_id** **release_id**  
     1               01
     2               15
     3               10
     4               20
     5               10

Please help me with the MySQL SQL to accomplish this. I tried doing a union of left and right joins but it did not work.

Answer 1

You could take the rows from table_1 and then union all them the extra rows from table_2 . Clunky, but should get the job done:

SELECT *
FROM   table_1
UNION ALL
SELECT *
FROM   table_2
WHERE  variable_id NOT IN (SELECT variable_id FROM table_1)

Answer 2

SELECT table_2.variable_id, 
       IFNULL(table_1.release_id, table_2.release_id) release_id
FROM table_2 
LEFT JOIN table_1 
ON table_2.variable_id = table_1.variable_id