我们如何通过LEFT JOIN在MySQL表中减去值？

mysql> Select * from value_curdate; +----+----------+-------+ | Id | Product | Price | +----+----------+-------+ | 1 | Notebook | 100 | | 2 | Pen | 40 | | 3 | Pencil | 65 | +----+----------+-------+ 3 rows in set (0.00 sec) mysql> Select * from value_prevdate; +----+-----------+-------+ | Id | Product | Price | +----+-----------+-------+ | 1 | Notebook | 85 | | 2 | Pen | 34 | | 3 | Pencil | 56 | | 4 | Colors | 65 | | 5 | Fevistick | 25 | +----+-----------+-------+ 5 rows in set (0.00 sec)

上述两个表分别存储了产品的当前价格和之前的价格。现在，通过使用LEFT JOIN，以下查询将找到存储在两个表中相同产品之间的价格差异。

mysql> Select value_curdate.id, value_curdate.product, value_curdate.price as Curprice,value_prevdate.price as 'prevprice', value_curdate.price-value_prevdate.price as 'Difference' from value_curdate LEFT JOIN value_prevdate ON value_curdate.id = value_prevdate.id ;
+----+----------+----------+-----------+------------+
| id | product  | Curprice | prevprice | Difference |
+----+----------+----------+-----------+------------+
| 1  | Notebook | 100      | 85        | 15         |
| 2  | Pen      | 40       | 34        | 6          |
| 3  | Pencil   | 65       | 56        | 9          |
+----+----------+----------+-----------+------------+
3 rows in set (0.00 sec)

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