mysql连表统计查询问题，求助

Answer 1

select u2.username as borrower,u1.username as debtor,sum(b.amount) as debt from borrow_log b left join users u1 on b.debtor = u1.id left join users u2 on b.borrower = u2.id where u1.gender = 1 group by b.borrower,b.debtor;

Answer 2

SELECT 
  (SELECT us.username FROM users us WHERE us.id=a.borrow) AS borrow_name,
  (SELECT us.username FROM users us WHERE us.id=a.debtor) AS debtor_name,
  SUM(amount) AS total_amount 
FROM
  borrow_log a 
  LEFT JOIN users b ON a.debtor = b.id 
WHERE b.gender = 1 
GROUP BY a.borrow,a.debtor

Answer 3

SELECT a1.*,a2.username AS debname,a3.username AS borrowname,SUM(a1.amount) FROM a1
LEFT JOIN a2 ON a1.debtor=a2.id
LEFT JOIN a2 AS a3 ON a1.borrower=a3.id
WHERE a2.gender=1
GROUP BY a1.debtor ,a1.borrower