假设您有这些表:
+----------------------------+
| PRODUCTS |
+------+-------------+-------+
| code | name | price |
+------+-------------+-------+
| 1 | Bike Helmet | 99.99 |
| 2 | Shirt | 19.99 |
+------+-------------+-------+
+-------------------+
| CATEGORIES |
+------+------------+
| code | category |
+------+------------+
| 1 | Sports |
| 1 | Cycling |
| 1 | Protection |
| 2 | Men |
| 2 | Clothing |
+------+------------+
这是一个基于另一个 SO 答案的查询,如果我对它的解释是正确的,它将符合您想要的结果:
SELECT p.code, p.name, p.prize,
(SELECT category FROM categories c WHERE c.code = p.code LIMIT 0, 1) as category1,
(SELECT category FROM categories c WHERE c.code = p.code LIMIT 1, 1) as category2,
(SELECT category FROM categories c WHERE c.code = p.code LIMIT 2, 1) as category3,
FROM products p
这是结果:
+------+-------------+-------+-----------+-----------+------------+
| code | name | price | category1 | category2 | category3 |
+------+-------------+-------+-----------+-----------+------------+
| 1 | Bike Helmet | 99.99 | sports | cycling | protection |
| 2 | Shirt | 19.99 | men | clothing | NULL |
+------+-------------+-------+-----------+-----------+------------+
但是,结果中不可能有动态数量的类别。在这种情况下,我将数字限制为 3,就像您的问题一样。可能仍然有性能更好的解决方案。此外,这显然是一个嵌套查询,因此可能不适合您的需求。不过,这是我能想到的最好的。
加入
还有 SQL JOIN子句,这可能是您要查找的内容:
SELECT *
FROM products
NATURAL JOIN categories
你最终会得到这个结果:
+------+-------------+-------+------------+
| code | name | price | category |
+------+-------------+-------+------------+
| 1 | Bike Helmet | 99.99 | sports |
| 1 | Bike Helmet | 99.99 | cycling |
| 1 | Bike Helmet | 99.99 | protection |
| 2 | Shirt | 19.99 | men |
| 2 | Shirt | 19.99 | clothing |
+------+-------------+-------+------------+