Sql 根据另一个表的值从另一个表中选择列

好吧，标题有点乱，但我有一个像这样的桌子城市：

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

现在我从另一个表中选择一个表：

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

问题是选择城市，因为城市是城市的名称，但我需要id。所以基本上我想要这样的东西，但这不起作用：

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

与city表进行内部联接，并在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 ;

---

从值中查找id有点不寻常。为什么要将城市存储在主表中？你应该把城市id存储在那里

但一个简单的连接就可以：

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

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