如何从 MySQL 中的两个表中减去相同的值？

Question

I have two tables with the same columns, but one table have more and different values than the other. How can I subtract the same values in two tables.

Table 1                Table 2
    Name   Number             Name  Number
    Dan    5                  Dan   2
    Jon    3                  Dev   1
    Dev    2
    Gin    2

How can I query to have the result:

    Name   Number            
    Dan    3                    
    Jon    3                    
    Dev    1
    Gin    2

Answer 1

You'll need a LEFT JOIN :

SELECT table1.name, 
        table1.num-IFNULL(table2.num,0)
FROM table1 LEFT JOIN table2
ON table1.name=table2.name;

IFNULL() is to replace NULL values with zero(0); returned from table on the right side of the LEFT JOIN ( table2 ) due to non-existing values.

Read more about MySQL JOIN .