选择基于另一个表的值从另一个表列

Question

好了，所以标题是有点乱，但我有一个表city这样的：

id | name  | population 
1 | Bla bla | 1323 
2 | Blaer | 35365 

现在，我选择到一个表从又另一个表：

INSERT INTO tmp (
    name, age, city_id 
) 
SELECT 
    NAME, 
    AGE, 
    CITY 
FROM main; 

的问题是SELECT CITY，因为CITY是城市的名字，但我需要的id。所以基本上我想是这样的，但是这并不工作：

INSERT INTO tmp (
    name, age, city_id 
) 
SELECT 
    NAME, 
    AGE, 
    city.id FROM city WHERE city.name = CITY 
FROM main; 

Answer 1

做一个INNER JOIN与城市表和SELECT子句中使用它。

INSERT INTO tmp (
    name, age, city_id 
) 
SELECT 
    NAME, 
    AGE, 
    c.id 
FROM main m 
INNER JOIN city c on m.city=c.name ; 

Answer 2

从价值中找到id有点不寻常。你为什么把城市存放在主表中？你应该在那里存储城市ID。

但一个简单连接就可以了：

INSERT INTO tmp (name, age, city_id) 
SELECT 
    m.NAME, 
    m.AGE, 
    c.id 
FROM main m join city c on m.city = c.name 

Answer 3

试试这个

INSERT INTO tmp(
    name, 
    age, 
    city_id 
) 
SELECT 
    m.name AS name, 
    m.age AS age, 
    (SELECT c.id FROM city c WHERE c.name = m.city) AS city_id 
FROM main m