如何使用列变量作为列名

Question

我目前有以下查询：

SELECT a.instance_type, SUM(a.quantity) as quantity, b.name, b.id
FROM sales_iteminstance a
INNER JOIN inventory_item b ON b.id = a.fk_item_id
GROUP BY (a.instance_type, b.id)
ORDER BY (b.id)

哪个returns:

+---------------+----------+----------+----+
| instance_type | quantity |   name   | id |
+---------------+----------+----------+----+
| Sell          |        5 | Gas 50Kg |  5 |
| Buy           |        8 | Gas 50Kg |  5 |
| Return        |        4 | Gas 50Kg |  5 |
+---------------+----------+----------+----+

是否可以更新查询以压缩上面的 table 并使用 instance_type 作为列名？即

+----+----------+------+-----+--------+
| id |   name   | sell | buy | return |
+----+----------+------+-----+--------+
|  5 | Gas 50Kg |    5 |   8 |      4 |
+----+----------+------+-----+--------+

Answer 1

是的，一个简单的数据透视查询可以做到这一点：

SELECT
    id,
    name,
    MAX(CASE WHEN instance_type = 'Sell'   THEN quantity END) AS sell,
    MAX(CASE WHEN instance_type = 'Buy'    THEN quantity END) AS buy,
    MAX(CASE WHEN instance_type = 'Return' THEN quantity END) AS "return"
FROM yourTable
GROUP BY
    id, name;

Answer 2

您也可以在 Postgres (9.4+) 中使用 FILTER 子句

SELECT
    id,
    name,
    MAX(quantity) FILTER ( WHERE instance_type = 'Sell' ) AS sell,
    MAX(quantity) FILTER ( WHERE instance_type = 'Buy'  ) AS buy,
    MAX(quantity) FILTER ( WHERE instance_type = 'Return') AS "return"
FROM yourTable
GROUP BY
    id, name;

Demo

如何使用列变量作为列名

How to use column variables as column names

sql

postgresql

pivot

crosstab