Recommend friend SQL based on two-degree connections

Friend table structure

DROP TABLE IF EXISTS FRIEND;
CREATE TABLE Friend (
UID bigint not NULL comment ' User ID ',
Friend_uid bigint NOT NULL comment ' apply to friend's user ID ',
Sys_create_date datetime NOT NULL comment ' Application time ',
Sys_last_update datetime NOT NULL comment ' Update time ',
State tinyint not null comment ' friend status 0: Delete, 1: Request friend, 2: Friend ',
Primary KEY (UID, friend_uid)
) Engine=innodb DEFAULT Charset=utf8 COMMENT ' Friends table ';

Select Friend_uid from

Friend where state=2 and
UID in (select Friend_uid from friend where Uid=13 and state=2)

and Friend_uid not in (select Friend_uid from friend where uid=13) and friend_uid! = Limit 6

UID is the ID of the current user, and a user with ID 13 is queried for two-degree friends