Invitation registration, SQL statements do not know how to write,

Invitation to register. I don't know how to write the SQL statement. for help !!!! The database is mysql, and Data Table A has four fields: & nbsp; id, name, password, and pidid. Pid is the id of the invitee at the time of registration. the default value is 0. four data IDs & nbsp; name invite registration. you do not know how to write the SQL statement, help !!!!
The database is mysql, and Data Table A contains four fields: id, name, password, and pid.

Id is the auto-increment primary key.

Pid is the id of the invitee at registration. the default value is 0.

Four data entries

Id name password pid
1 test 121121 0
2 Test 2 212121 1
3 Test 3 415454 1
4 Test 4 1212121 3

How can I use SQL statements to show who invited me? How many people have you invited?
------ Solution --------------------
Not tested. Adjust or ask again

Select u. id, count (ui. id), group_concat (ui. id)
From users u
Join users ui
On u. id = ui. pid
Group by u. id
;