sql 怎么把不满足条件的显示为null

这是左外连接产生的结果，就是采用左表所有记录，匹配右表，若右表没有相应记录，则以NULL显示
大致的SQL语句如下：
SELECT A.sid, A.sname, t.money, t.amount
FROM A left outer join
(SELECT sid, sum(交易金额) as money, sum(交易量) as amount
FROM B WHERE 交易时间 BETWEEN(。。。，。。。)
GROUP BY sid) t
ON A.sid = t.sid

给你个思路
SELECT t.sid， t.sname ， (SELECT CASE WHEN (t.总金额< 45) THEN (null)
ELSE (t.总金额) END AS Expr1) AS 总金额),
(SELECT CASE WHEN (t.总量<2) THEN (null)
ELSE (t.总量) END AS Expr1) AS 总量)
FROM t