在select子句中重用子查询可以通过使用内联视图或者公用表表达式(CTE)来实现。
例如,假设我们有一个订单表和一个订单详情表,我们想要查询每个订单的总金额和平均金额。可以使用内联视图来实现:
SELECT
o.order_id,
o.customer_id,
(SELECT SUM(quantity * price) FROM order_details WHERE order_id = o.order_id) AS total_amount,
(SELECT AVG(quantity * price) FROM order_details WHERE order_id = o.order_id) AS average_amount
FROM
orders o;
在上面的例子中,我们在select子句中使用了两个子查询来计算总金额和平均金额,并将它们作为表达式使用。
使用CTE来重用子查询的语法如下:
WITH cte_name AS (
SELECT column1, column2, ...
FROM table_name
WHERE condition
)
SELECT
cte_name.column1,
cte_name.column2,
...
FROM
cte_name;
例如,我们可以使用CTE来重用上面的例子中的子查询:
WITH order_amounts AS (
SELECT
order_id,
SUM(quantity * price) AS total_amount,
AVG(quantity * price) AS average_amount
FROM
order_details
GROUP BY
order_id
)
SELECT
o.order_id,
o.customer_id,
oa.total_amount,
oa.average_amount
FROM
orders o
JOIN
order_amounts oa ON o.order_id = oa.order_id;
在上面的例子中,我们首先定义了一个CTE(order_amounts),它计算了每个订单的总金额和平均金额。然后,在主查询中,我们将CTE与订单表进行连接,以获取所需的结果。
总结:
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