SQL查询从另一个表中获取数据

Question

I've been trying to build a little complex SQL query to fetch some keys from a table which has no relation with the one in which I need to "ask".

Practical case:

I have four tables: users, devices, preferences and user_devices.

• Table users, there are user information and the user "ID".
• Table devices, there are columns with device information, the device "ID" (device ID) and the device "key" (device internal ID).
• Table preferences, "user ID" and some boolean fields with weekdays (monday, tuesday, wednesday...).
• Table user_devices, assign ID (auto_increment, not important), "user_id" and "device_id" with relations to their respective tables. This tabble is used to assign a device to a user.

The thing is that I need to obtain all the "devices.key" of users that have the preference "monday" set to true in table preferences.

Example: I ask the database for devices with preference monday true and I obtain an array with two device key.

This query fetch the device key of the user 4, but I need to fetch de device key of all the users with "monday" set to true (1) in the database.

SELECT key FROM devices, user_devices WHERE devices.id = user_devices.device_id AND user_devices.user_id = 4

Thank you very much.

Answer 1

you can try following query

SELECT `devices`.`key` 
FROM users 
JOIN preferences ON users.id = preferences.user_id
JOIN user_devices ON users.id = user_devices.user_id
JOIN devices ON user_devices.device_id = devices.id
WHERE preferences.monday = true

obviously you'll have to check and correct the column names of preferences and users table in query to match your database column names.