选择语句

SELECT p.id,
       p.product_name,
       p.product_date,
       coalesce(t.unique_hits, 0) AS unique_hits,
       coalesce(t.total_hits, 0) AS total_hits,
       coalesce(COUNT(s.product_id), 0) AS total_sales,
       coalesce(SUM(s.amount), 0) AS total_revenue
FROM products p
LEFT OUTER JOIN tracking_hits t ON p.id = t.product_id
LEFT OUTER JOIN transactions s ON p.id = s.product_id
WHERE p.vendor_id = 10;

DDL示例：
http://sqlfiddle.com/#!2/b393df/3

我添加了两个具有相同供应商ID的产品，当我运行查询时，它只选择一个产品，即使它应该同时选择它们。

用于创建表和数据的SQL

CREATE TABLE products(id int, product_name varchar(100)
      , product_date varchar(100), vendor_id int);

CREATE TABLE tracking_hits(id int, product_id int, unique_hits int
      , total_hits int);

CREATE TABLE transactions(id int, product_id int, amount double);

插入一些数据

INSERT INTO products(id, product_name, product_date, vendor_id)
VALUES(0, "Product", "2014-05-02", 10);

INSERT INTO products(id, product_name, product_date, vendor_id)
VALUES(1, "Product", "2014-05-02", 10);

INSERT INTO tracking_hits(id, product_id, unique_hits, total_hits)
VALUES(0, 0, 10, 52);

INSERT INTO tracking_hits(id, product_id, unique_hits, total_hits)
VALUES(1, 1, 52, 124);    

INSERT INTO transactions(id, product_id, amount)
VALUES(0, 0, 19.00);

INSERT INTO transactions(id, product_id, amount)
VALUES(1, 0, 19.00);

INSERT INTO transactions(id, product_id, amount)
VALUES(2, 0, 19.00);

INSERT INTO transactions(id, product_id, amount)
VALUES(3, 1, 29.00);

这里的问题是什么？

Answer 1

您在执行聚合函数sum()

时缺少分组

SELECT p.id,
       p.product_name,
       p.product_date,
       coalesce(t.unique_hits, 0) AS unique_hits,
       coalesce(t.total_hits, 0) AS total_hits,
       coalesce(COUNT(s.product_id), 0) AS total_sales,
       coalesce(SUM(s.amount), 0) AS total_revenue
FROM products p
LEFT OUTER JOIN tracking_hits t ON p.id = t.product_id
LEFT OUTER JOIN transactions s ON p.id = s.product_id
WHERE p.vendor_id = 10
group by p.id
;

Answer 2

以下是您的查询：

SELECT p.id,
       p.product_name,
       p.product_date,
       coalesce(t.unique_hits, 0) AS unique_hits,
       coalesce(t.total_hits, 0) AS total_hits,
       coalesce(COUNT(s.product_id), 0) AS total_sales,
       coalesce(SUM(s.amount), 0) AS total_revenue
FROM products p
LEFT OUTER JOIN tracking_hits t ON p.id = t.product_id
LEFT OUTER JOIN transactions s ON p.id = s.product_id
WHERE p.vendor_id = 10;

这是一个没有group by的聚合查询（因为在sum()中使用了聚合函数count()和select）。这样的查询总是返回一行。我想你想要：

SELECT p.id,
       p.product_name,
       p.product_date,
       coalesce(t.unique_hits, 0) AS unique_hits,
       coalesce(t.total_hits, 0) AS total_hits,
       coalesce(COUNT(s.product_id), 0) AS total_sales,
       coalesce(SUM(s.amount), 0) AS total_revenue
FROM products p
LEFT OUTER JOIN tracking_hits t ON p.id = t.product_id
LEFT OUTER JOIN transactions s ON p.id = s.product_id
WHERE p.vendor_id = 10
GROUP BY p.id, t.unique_hits, t.total_hits;

Answer 3

基本问题是在使用GROUP BY和SUM等聚合时缺少COUNT。除此之外，我认为你可以通过避免所有coalesce函数来简化你的查询。 DBMS应该足够聪明，可以将null值作为0处理，而无需明确告知它。如果你想在0之间拥有不同的价值，情况会发生变化。这是一个工作（sqlfiddle）示例：

SELECT p.id,
       p.product_name,
       p.product_date,
       t.unique_hits AS unique_hits,
       t.total_hits AS total_hits,
       COUNT(s.product_id) AS total_sales,
       SUM(s.amount) AS total_revenue
FROM products p
LEFT JOIN tracking_hits t ON p.id = t.product_id
LEFT JOIN transactions s ON p.id = s.product_id
WHERE p.vendor_id = 10
GROUP BY p.id, p.product_name, p.product_date, t.unique_hits, t.total_hits;

更新：我也t.unique_hits, t.total_hits添加了GROUP BY。

带连接的SQL查询仅选择1行

选择语句

用于创建表和数据的SQL

3 个答案: