【用于数据分析的 SQL（四）】【SQL 子槽和临时表】

一、 前言

学习嵌套查询（子查询），解决复杂的业务问题

二、 子查询

使用子查询时要记住的一点是为读者提供某种方式，轻松确定查询的哪些部分将一起执行。大多数人通过以某种方式缩定子问题来做到这一点

当在有条件的语句中写子问题时，不应包含别名

三、 与

1.查找每个频道每天的平均事件数

SELECT channel, AVG(events) AS average_events
FROM (SELECT DATE_TRUNC('day',occurred_at) AS day,
             channel, COUNT(*) as events
      FROM web_events 
      GROUP BY 1,2) sub
GROUP BY channel
ORDER BY 2 DESC;

使用与语句

WITH events AS (
          SELECT DATE_TRUNC('day',occurred_at) AS day, 
                        channel, COUNT(*) as events
          FROM web_events 
          GROUP BY 1,2)

SELECT channel, AVG(events) AS average_events
FROM events
GROUP BY channel
ORDER BY 2 DESC;

2.提供销售额最大的地区sales_rep的名称，total_amt_usd销售额最大。

WITH t1 AS (
  SELECT s.name rep_name, r.name region_name, SUM(o.total_amt_usd) total_amt
   FROM sales_reps s
   JOIN accounts a
   ON a.sales_rep_id = s.id
   JOIN orders o
   ON o.account_id = a.id
   JOIN region r
   ON r.id = s.region_id
   GROUP BY 1,2
   ORDER BY 3 DESC), 
t2 AS (
   SELECT region_name, MAX(total_amt) total_amt
   FROM t1
   GROUP BY 1)
SELECT t1.rep_name, t1.region_name, t1.total_amt
FROM t1
JOIN t2
ON t1.region_name = t2.region_name AND t1.total_amt = t2.total_amt;

3.对于销售total_amt_usd最多的地区，总订单数是多少？

WITH t1 AS (
   SELECT r.name region_name, SUM(o.total_amt_usd) total_amt
   FROM sales_reps s
   JOIN accounts a
   ON a.sales_rep_id = s.id
   JOIN orders o
   ON o.account_id = a.id
   JOIN region r
   ON r.id = s.region_id
   GROUP BY r.name), 
t2 AS (
   SELECT MAX(total_amt)
   FROM t1)
SELECT r.name, COUNT(o.total) total_orders
FROM sales_reps s
JOIN accounts a
ON a.sales_rep_id = s.id
JOIN orders o
ON o.account_id = a.id
JOIN region r
ON r.id = s.region_id
GROUP BY r.name
HAVING SUM(o.total_amt_usd) = (SELECT * FROM t2);

4.有多少账户的总购买量超过了作为客户在其一生中购买最standard_qty纸的账户名称？

WITH t1 AS (
  SELECT a.name account_name, SUM(o.standard_qty) total_std, SUM(o.total) total
  FROM accounts a
  JOIN orders o
  ON o.account_id = a.id
  GROUP BY 1
  ORDER BY 2 DESC
  LIMIT 1), 
t2 AS (
  SELECT a.name
  FROM orders o
  JOIN accounts a
  ON a.id = o.account_id
  GROUP BY 1
  HAVING SUM(o.total) > (SELECT total FROM t1))
SELECT COUNT(*)
FROM t2;

5.对于花费最多（在客户一生中）total_amt_usd的客户来说，他们每个渠道有多少web_events？

WITH t1 AS (
   SELECT a.id, a.name, SUM(o.total_amt_usd) tot_spent
   FROM orders o
   JOIN accounts a
   ON a.id = o.account_id
   GROUP BY a.id, a.name
   ORDER BY 3 DESC
   LIMIT 1)
SELECT a.name, w.channel, COUNT(*)
FROM accounts a
JOIN web_events w
ON a.id = w.account_id AND a.id =  (SELECT id FROM t1)
GROUP BY 1, 2
ORDER BY 3 DESC;

6.前10个总支出账户的total_amt_usd寿命平均支出是多少？

WITH t1 AS (
   SELECT a.id, a.name, SUM(o.total_amt_usd) tot_spent
   FROM orders o
   JOIN accounts a
   ON a.id = o.account_id
   GROUP BY a.id, a.name
   ORDER BY 3 DESC
   LIMIT 10)
SELECT AVG(tot_spent)
FROM t1;

7.以total_amt_usd计算的终身平均花费是多少，只包括每个订单平均花费比所有订单平均花费更多的公司。

WITH t1 AS (
   SELECT AVG(o.total_amt_usd) avg_all
   FROM orders o
   JOIN accounts a
   ON a.id = o.account_id),
t2 AS (
   SELECT o.account_id, AVG(o.total_amt_usd) avg_amt
   FROM orders o
   GROUP BY 1
   HAVING AVG(o.total_amt_usd) > (SELECT * FROM t1))
SELECT AVG(avg_amt)
FROM t2;

子查询和WITH的先进功能是公司内部分析角色中最广泛的应用。能够将问题分解到必要的表中，并使用生成的表找到解决方案在实践中非常有用