在 Redshift 中使用 CTE

Redshift 入门

Jason Myers

Principal Architect

公共表表达式（CTE）

临时结果集
简化查询
子查询的替代方案

子查询与 CTE 结构

SELECT division_name,
       revenue_total
       -- Subquery for top 10 divions by revenue
  FROM (SELECT division_id,
               SUM(revenue) AS revenue_total
          FROM orders
         GROUP BY division_id
         ORDER BY revenue_total DESC
         LIMIT 10) top_ten_divisions_by_rev
 WHERE revenue_total > 100000;

-- CTE for top 10 divions by revenue
WITH top_ten_divisions_by_rev AS (
  SELECT division_id,
         SUM(revenue) AS revenue_total
    FROM orders
   GROUP BY division_id
   ORDER BY revenue_total DESC
   LIMIT 10)


-- Main query
SELECT division_id,
       revenue_total
       -- FROM our CTE
  FROM top_ten_divisions_by_rev
 WHERE revenue_total > 100000;

多个 CTE

-- Top 10 divisions by revenue CTE
WITH top_ten_divisions_by_rev AS(
  SELECT division_id,
         SUM(revenue) AS revenue_total
    FROM orders
   GROUP BY division_id
   ORDER BY revenue_total DESC
   LIMIT 10),

-- Division ID and Name CTE
division_names AS(
  SELECT id AS division_id,
         name AS division_name
    FROM divisions)

-- Main query
SELECT division_name,
       revenue_total
       -- FROM Top 10 CTE
  FROM top_ten_divisions_by_rev
  -- Joining so we can use the division name
  JOIN division_names USING (DIVISION_ID)
 WHERE revenue_total > 100000;

多个 CTE 与连接

```sql
-- Selecting the division names
WITH division_names AS(
  SELECT id AS division_id,
         name AS division_name
    FROM divisions),
-- Selecting the top ten divisions by revenue
top_ten_divisions_by_rev AS(
  SELECT division_name,
         SUM(revenue) AS revenue_total
    FROM orders
    -- Joining in the division_names CTE
    JOIN divisions USING (division_id)
   GROUP BY division_name
   ORDER BY revenue_total DESC
   LIMIT 10);

克尼汉法则

众所周知，调试比最初编写程序要难两倍。因此，如果你在编写时已经尽可能巧妙，那你又如何调试它？

CTE 性能

与子查询相同
若复用，优于子查询

SQL 代码与执行顺序

让我们练习！

Redshift 入门