Problem

(0) sql :       expand: "%{User-Name}" -> 'demo'
(0) sql : SQL-User-Name set to 'demo'
rlm_sql (sql): Reserved connection (4)
(0) sql :       expand: "SELECT id, username, attribute, value, op FROM radcheck WHERE username = '%{SQL-User-Name}' ORDER BY id" -> 'SELECT id, username, attribute, value, op FROM radcheck WHERE username = 'demo' ORDER BY id'
rlm_sql (sql): Executing query: 'SELECT id, username, attribute, value, op FROM radcheck WHERE username = 'demo' ORDER BY id'
(0) sql :       expand: "SELECT groupname FROM radusergroup WHERE username = '%{SQL-User-Name}' ORDER BY priority" -> 'SELECT groupname FROM radusergroup WHERE username = 'demo' ORDER BY priority'
rlm_sql (sql): Executing query: 'SELECT groupname FROM radusergroup WHERE username = 'demo' ORDER BY priority'
rlm_sql (sql): Released connection (4)
rlm_sql (sql): Closing connection (0): Too many free connections (5 > 3)
(0)   [sql] = notfound

Solution

Make sure the driver is correct in the enabled module – sql

vim /etc/raddb/mods-enabled/sql
driver = "rlm_sql_mysql"