MySQL JOIN最近的行而已？

Question

我有一个表客户，存储有customer_id，电子邮件和参考。还有一个附加表customer_data，用于存储对客户所做的更改的历史记录，即，当发生更改时，将插入新行。

为了在一个表中显示客户信息，需要将两个表连接起来，但是只有customer_data中的最新行才应该连接到客户表中。

由于查询是分页的，因此变得有点复杂，因此具有限制和偏移量。

我该如何使用MySQL？我想我想在某个地方放一个DISTINCT …

此刻的查询是这样的-

SELECT *, CONCAT(title,' ',forename,' ',surname) AS name
FROM customer c
INNER JOIN customer_data d on c.customer_id=d.customer_id
WHERE name LIKE '%Smith%' LIMIT 10, 20

另外，我是否认为可以通过这种方式将CONCAT与LIKE一起使用？

（我很欣赏INNER JOIN可能是错误的JOIN类型。实际上我不知道不同的JOIN之间有什么区别。我现在要研究一下！）

Answer 1

您可能需要尝试以下方法：

SELECT    CONCAT(title, ' ', forename, ' ', surname) AS name
FROM      customer c
JOIN      (
              SELECT    MAX(id) max_id, customer_id 
              FROM      customer_data 
              GROUP BY  customer_id
          ) c_max ON (c_max.customer_id = c.customer_id)
JOIN      customer_data cd ON (cd.id = c_max.max_id)
WHERE     CONCAT(title, ' ', forename, ' ', surname) LIKE '%Smith%' 
LIMIT     10, 20;

请注意，a JOIN只是的同义词INNER JOIN。

测试用例：

CREATE TABLE customer (customer_id int);
CREATE TABLE customer_data (
   id int, 
   customer_id int, 
   title varchar(10),
   forename varchar(10),
   surname varchar(10)
);

INSERT INTO customer VALUES (1);
INSERT INTO customer VALUES (2);
INSERT INTO customer VALUES (3);

INSERT INTO customer_data VALUES (1, 1, 'Mr', 'Bobby', 'Smith');
INSERT INTO customer_data VALUES (2, 1, 'Mr', 'Bob', 'Smith');
INSERT INTO customer_data VALUES (3, 2, 'Mr', 'Jane', 'Green');
INSERT INTO customer_data VALUES (4, 2, 'Miss', 'Jane', 'Green');
INSERT INTO customer_data VALUES (5, 3, 'Dr', 'Jack', 'Black');

结果（不含LIMIT和的查询WHERE）：

SELECT    CONCAT(title, ' ', forename, ' ', surname) AS name
FROM      customer c
JOIN      (
              SELECT    MAX(id) max_id, customer_id 
              FROM      customer_data 
              GROUP BY  customer_id
          ) c_max ON (c_max.customer_id = c.customer_id)
JOIN      customer_data cd ON (cd.id = c_max.max_id);

+-----------------+
| name            |
+-----------------+
| Mr Bob Smith    |
| Miss Jane Green |
| Dr Jack Black   |
+-----------------+
3 rows in set (0.00 sec)