Mysql - 如果记录在第三个表中，则连接 2 个表

Question

I have 3 tables

Table 1 = Customers

CREATE TABLE `Customers` (
`customer_id` int NOT NULL,
`name` varchar(5) NOT NULL
)

Table 2 = Wallposts

CREATE TABLE `wallposts` (
`customer_id` int NOT NULL,
`post` varchar(255) NOT NULL
)

Table 3 = Friend_list

CREATE TABLE `friends` (
`customer_id` int NOT NULL,
`friend_id` int NOT NULL
)

I need to get a list of all wallposts by friends of a specific customer_id. The friend_id is taken from the customer_id in the customers table

Example output

customer_id 2, name John, message hello world
customer_id 4, name Susan, message Customer service sucks
customer_id 7, name Philip, message Customer service is great

All those posts belong to friends of customer with customer_id 1.

Answer 1

A series of joins should do the trick

SELECT c.customer_id, c.name, w.post
FROM   friends f
JOIN   customer c ON f.friend_id = c.customer_id
JOIN   wallposts w ON c.customer_id = w.customer_id
WHERE  f.customer_id = 1

Answer 2

Another way to do it can be as below

SELECT
    customers.customer_id, customers.name, wallposts.posts
FROM
    wallposts
INNER JOIN customers ON wallposts.customer_id = customers.customer_id
WHERE wallposts.customer_id IN(SELECT friend_id FROM friends WHERE customer_id = 1)