mysql自然连接整理

SELECT
    realname,
    a.bj_id,
    a.uid,
    a.pid,
    CASE
WHEN edate IS NULL
AND sdate IS NOT NULL THEN
    '正在考试'
WHEN sdate IS NOT NULL
AND edate IS NOT NULL THEN
    '已经考完'
WHEN sdate IS NULL
AND edate IS NULL THEN
    '缺考学生'
END kszt,
 paper_name,
 starttime,
 endtime,
 sdate,
 edate,
 total_score,
 score,
 ip,
 userno
FROM
    (
        SELECT
            t1.id AS uid,
            t1.realname,
            t1.bj_id,
            t3.id AS pid,
            t3.paper_name,
            t3.starttime,
            t3.endtime,
            t3.total_score,
            t1.userno
        FROM
            tm_user t1,
            tm_paper_class t2,
            tm_paper t3
        WHERE
            t1.bj_id = t2.bjid
        AND t2.paperid = t3.id
        AND t2.paperid = 237
    ) a
LEFT JOIN tm_exam_info b ON a.uid = b.uid
AND b.pid = 237;