Question

你能帮我构建一个SQL查询来获取2表中的数据列表，如果第二个表中存在数据，它应该显示来自它的数据，否则来自table1

表1：

ID | NAME    | AGE 
------------------
1  | John    | 20
2  | Daniel  | 30
3  | Abraham | 30
4  | Donald  | 25

表2：

ID | NAME    | AGE 
------------------
1  | John    | 23
2  | Donald  | 24

期望的结果：约翰和唐纳德＆＃34; 年龄＆＃34;从表2中获取数据，其余来自表1

ID | NAME    | AGE 
------------------
1  | John    | 23
2  | Daniel  | 30
3  | Abraham | 30
4  | Donald  | 24

Answer 1

使用自动增量ID加入的表

SELECT t1.id, t1.name, IF(t2.id, t2.age, t1.age) AS age
FROM Clients AS t1
LEFT JOIN Records AS t2 ON (t1.id = t2.id)

或使用名称

加入的表格

SELECT t1.id, t1.name, IF(t2.name, t2.age, t1.age) AS age
FROM Clients AS t1
LEFT JOIN Records AS t2 ON (t1.name = t2.name)

Answer 2

记录的外部联接，并测试age的NULL值。像这样：

SELECT c.id
     , c.name
     , IFNULL(r.age,c.age) AS age
  FROM Clients c
  LEFT
  JOIN Records r
    ON r.name = c.name
 ORDER BY c.id