MySQL的加入两个表为一个

Question

我有两个虚表：

1. 人民

id, age 
1, 25 

2. 名称

id, people_id, type, value 
1, 1, 'first', 'John' 
2, 1, 'last', 'Doe' 

我如何加入两个表，所以我'd得到这个结果：

id, age, first, last 
1, 25, 'John', 'Doe' 

Answer 1

SELECT a.ID, 
     a.Age, 
     MAX(CASE WHEN b.type = 'first' THEN b.Value END) `First`, 
     MAX(CASE WHEN b.type = 'last' THEN b.Value END) `Last` 
FROM People a 
     LEFT JOIN Names b 
      ON a.ID = b.people_id 
GROUP BY a.ID, a.Age 

• SQLFiddle Demo

否则，如果您有type列未知值，动态声明更加优选。

SET @sql = NULL; 

SELECT GROUP_CONCAT(DISTINCT 
     CONCAT('MAX(CASE WHEN b.type = ''', 
       type, 
       ''' THEN value ELSE NULL END) AS ', 
       CONCAT('`', type, '`') 
       )) INTO @sql 
FROM Names; 

SET @sql = CONCAT('SELECT a.ID, 
          a.Age, ', @sql , ' 
        FROM People a 
          LEFT JOIN Names b 
           ON a.ID = b.people_id 
        GROUP BY a.ID, a.Age'); 

PREPARE stmt FROM @sql; 
EXECUTE stmt; 
DEALLOCATE PREPARE stmt; 

• SQLFiddle Demo

Answer 2

您正在寻找如何加入来自不同表的数据的一般信息。它被称为内连接。 参考可以发现here。

Answer 3

希望它可以帮助你，

方法：1

select p.id, p.age, n.type as first, n.value as last from People as p, Names as n where p.id = n.people_id 

方法：2

select p.id, p.age, n.type as first, n.value as last from People as p LEFT JOIN Names as n ON p.id = n.people_id 

Answer 4

SELECT p.id, p.age, n.first, n.last 
FROM People p 
JOIN Names n 
ON p.id = n.people_id;