SQL查询连接三个表

CREATE TABLE customer (
  customer_id mediumint(8) unsigned NOT NULL auto_increment,name varchar(50) NOT NULL,PRIMARY KEY (customer_id)
);

INSERT INTO customer VALUES (1,'Dagmar');
INSERT INTO customer VALUES (2,'Dietmar');
INSERT INTO customer VALUES (3,'Sabine');

CREATE TABLE sales_cars (
  sale_id mediumint(8) unsigned NOT NULL auto_increment,customer_id mediumint(8) unsigned NOT NULL,sale_amount decimal(10,2) NOT NULL,PRIMARY KEY (sale_id)
);

INSERT INTO sales_cars VALUES (1,3,14.40);
INSERT INTO sales_cars VALUES (2,1,28.30);
INSERT INTO sales_cars VALUES (3,2,34.40);
INSERT INTO sales_cars VALUES (4,25.60);

CREATE TABLE sales_parts (
  sale_id mediumint(8) unsigned NOT NULL auto_increment,PRIMARY KEY (sale_id)
);

INSERT INTO sales_parts VALUES (1,68.20);
INSERT INTO sales_parts VALUES (2,21.30);
INSERT INTO sales_parts VALUES (3,54.40);
INSERT INTO sales_parts VALUES (4,35.70);

sales_car和sales_parts保持客户的销售额.我们的想法是编写一个查询,该查询将特定客户的汽车和零件的“sale_amount”相加,并按结果对结果进行分组.

有人有一个建议我怎么能解决这个问题？

解决方法

SELECT  c.customer_id,tot_cars.total + tot_parts.total AS total_sales
FROM    customer c
JOIN    (
           SELECT   customer_id,SUM(sale_amount) total
           FROM     sales_cars
           GROUP BY customer_id
        ) tot_cars ON (tot_cars.customer_id = c.customer_id)
JOIN    (
           SELECT   customer_id,SUM(sale_amount) total
           FROM     sales_parts
           GROUP BY customer_id
        ) tot_parts ON (tot_parts.customer_id = c.customer_id);

结果：

+-------------+-------------+
| customer_id | total_sales |
+-------------+-------------+
|           1 |       64.00 |
|           2 |      128.20 |
|           3 |       90.10 |
+-------------+-------------+
3 rows in set (0.03 sec)

更新：继续下面的评论：

让我们从sale_date字段开始：

CREATE TABLE sales_cars (
  sale_id mediumint(8) unsigned NOT NULL auto_increment,sale_date datetime NOT NULL,14.40,'2010-07-01 12:00:00');
INSERT INTO sales_cars VALUES (2,28.30,'2010-07-05 12:00:00');
INSERT INTO sales_cars VALUES (3,34.40,'2010-07-10 12:00:00');
INSERT INTO sales_cars VALUES (4,25.60,'2010-07-20 12:00:00');

要获取每个客户的最新销售日期,您可以将先前描述的查询与另一个派生表一起加入,如下所示：

SELECT  c.customer_id,tot_cars.total + tot_parts.total AS total_sales,latest_sales.date AS latest_sale
FROM    customer c
JOIN    (
           SELECT   customer_id,SUM(sale_amount) total
           FROM     sales_parts
           GROUP BY customer_id
        ) tot_parts ON (tot_parts.customer_id = c.customer_id)
JOIN    (
           SELECT   customer_id,MAX(sale_date) date
           FROM     sales_cars
           GROUP BY customer_id
        ) latest_sales ON (latest_sales.customer_id = c.customer_id);

结果：

+-------------+-------------+---------------------+
| customer_id | total_sales | latest_sale         |
+-------------+-------------+---------------------+
|           1 |       64.00 | 2010-07-05 12:00:00 |
|           2 |      128.20 | 2010-07-20 12:00:00 |
|           3 |       90.10 | 2010-07-01 12:00:00 |
+-------------+-------------+---------------------+
3 rows in set (0.07 sec)

你看到了这种模式吗？还有其他方法可以解决同样的问题,但加入派生表是一种非常简单直接的技术.

然后关于客户表中的更改,我假设您的意思是这样的：

CREATE TABLE customer (
  customer_id mediumint(8) unsigned NOT NULL auto_increment,first_name varchar(50) NOT NULL,last_name varchar(50) NOT NULL,gender char(1) NOT NULL,'Joe','Doe','M');
INSERT INTO customer VALUES (2,'Jane','Smith','F');
INSERT INTO customer VALUES (3,'Peter','Brown','M');

要在MySQL中连接字符串字段,您只需使用CONCAT()函数：

SELECT CONCAT(c.first_name,' ',c.last_name) as full_name
FROM   customer c;

返回：

+-------------+
| full_name   |
+-------------+
| Jane Smith  |
| Peter Brown |
| Joe Doe     |
+-------------+
3 rows in set (0.01 sec)

要有条件地应用“先生”或“女士”,您可以使用CASE声明：

SELECT (CASE c.gender WHEN 'M' THEN 'Mr' WHEN 'F' THEN 'Ms' END) salutaiton,CONCAT(c.first_name,c.last_name) as full_name
FROM   customer c;

返回：

+------------+-------------+
| salutaiton | full_name   |
+------------+-------------+
| Ms         | Jane Smith  |
| Mr         | Peter Brown |
| Mr         | Joe Doe     |
+------------+-------------+
3 rows in set (0.01 sec)

您还可以将两个字段连接在一起：

SELECT CONCAT((CASE c.gender WHEN 'M' THEN 'Mr' WHEN 'F' THEN 'Ms' END),c.first_name,c.last_name) as full_name
FROM   customer c;

返回：

+----------------+
| full_name      |
+----------------+
| Ms Jane Smith  |
| Mr Peter Brown |
| Mr Joe Doe     |
+----------------+
3 rows in set (0.00 sec)

最后,我们可以将其附加到我们的主查询中,CONCAT((CASE c.gender WHEN 'M' THEN 'Mr' WHEN 'F' THEN 'Ms' END),c.last_name) as full_name,MAX(sale_date) date FROM sales_cars GROUP BY customer_id ) latest_sales ON (latest_sales.customer_id = c.customer_id);

返回：

+-------------+----------------+-------------+---------------------+
| customer_id | full_name      | total_sales | latest_sale         |
+-------------+----------------+-------------+---------------------+
|           1 | Mr Joe Doe     |       64.00 | 2010-07-05 12:00:00 |
|           2 | Ms Jane Smith  |      128.20 | 2010-07-20 12:00:00 |
|           3 | Mr Peter Brown |       90.10 | 2010-07-01 12:00:00 |
+-------------+----------------+-------------+---------------------+
3 rows in set (0.02 sec)

（编辑：李大同）

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