请检查我的MYSQL查询并给我建议？

Question

这是我的mysql查询.

SELECT s.s_nric                           AS nric,
       s.s_name                           AS name,
       s.s_psle_eng                       AS psle_eng,
       s.s_psle_math                      AS psle_maths,
       s.s_psle_aggr                      AS psle_aggr,
       (SELECT re.re_mark
        FROM   si_results re
        WHERE  re.re_code LIKE 'FEEN%'
               AND re.re_year = '2008'
               AND re.re_semester = '2'
               AND re.re_nric = s.s_nric) AS english_2008,
       (SELECT re.re_mark
        FROM   si_results re
        WHERE  re.re_code LIKE 'FEMA%'
               AND re.re_year = '2008'
               AND re.re_semester = '2'
               AND re.re_nric = s.s_nric) maths_2008,
       (SELECT re.re_mark
        FROM   si_results re
        WHERE  re.re_code LIKE 'FEEN%'
               AND re.re_year = '2009'
               AND re.re_semester = '2'
               AND re.re_nric = s.s_nric) AS english_2009,
       (SELECT re.re_mark
        FROM   si_results re
        WHERE  re.re_code LIKE 'FEMA%'
               AND re.re_year = '2009'
               AND re.re_semester = '2'
               AND re.re_nric = s.s_nric) maths_2009,
       isc.isc_g_gpa                      AS isc_gpa
FROM   si_student_data AS s
       LEFT JOIN si_isc_gpa AS isc
         ON isc.isc_g_nric = s.s_nric
WHERE  1 = 1
       AND s.s_admission_year = '2008'
GROUP  BY s.s_nric
ORDER  BY s.s_gender,
          s.s_name ASC 

请检查我的子查询

这是我的子查询

(SELECT re.re_mark 
  FROM si_results re 
  WHERE re.re_code like 'FEEN%' 
    AND re.re_year='2008' 
    AND re.re_semester='2' 
    AND re.re_nric=s.s_nric) as English_2008, 
(SELECT re.re_mark 
  FROM si_results re 
  WHERE re.re_code like 'FEMA%' 
    AND re.re_year='2008' 
    AND re.re_semester='2' 
    AND re.re_nric=s.s_nric) Maths_2008, 
(SELECT re.re_mark 
  FROM si_results re 
  WHERE re.re_code like 'FEEN%' 
    AND re.re_year='2009' 
    AND re.re_semester='2' 
    AND re.re_nric=s.s_nric ) as English_2009, 
(SELECT re.re_mark 
  FROM si_results re 
  WHERE re.re_code like 'FEMA%' 
    AND re.re_year='2009' 
    AND re.re_semester='2' 
    AND re.re_nric=s.s_nric ) Maths_2009

当我执行查询时,服务器需要很长时间才能执行.那怎么简单呢？请指教.

谢谢.

Answer 1

重要的是要知道,LIKE 'XXXX%'如果没有索引,那么做一个非常慢.在某些情况下,这可能需要永远.此外,你正在做四个子选择而不是连接,这又慢了.下面我添加了一个完整的查询,它将产生相同的结果.替代方法是通过减少连接数来减小查询的大小,而是使用NORMALIZED输出.这适用于每一个nric,name你将拥有多行而不是re_mark每种类型的类(英语,数学,2008年,2009年等),你将有两列包含代码(FEEN或FEMA)和标记.

尝试以下查询,看看它是否适合您:

SELECT s.s_nric                           AS nric,
       s.s_name                           AS name,
       s.s_psle_eng                       AS psle_eng,
       s.s_psle_math                      AS psle_maths,
       s.s_psle_aggr                      AS psle_aggr,
       e_2008_feen.re_mark                AS english_2008,
       e_2008_fema.re_mark                AS maths_2008,
       e_2009_feen.re_mark                AS english_2009,
       e_2009_fema.re_mark                AS maths_2009,
       isc.isc_g_gpa                      AS isc_gpa
FROM   si_student_data AS s
INNER JOIN si_results e_2008_feen
    ON e_2008_feen.re_code LIKE 'FEEN%'
       AND e_2008_feen.re_year = '2008'
       AND e_2008_feen.re_semester = '2'
       AND e_2008_feen.re_nric = s.s_nric

INNER JOIN si_results e_2008_fema
    ON e_2008_fema.re_code LIKE 'FEMA%'
       AND e_2008_fema.re_year = '2008'
       AND e_2008_fema.re_semester = '2'
       AND e_2008_fema.re_nric = s.s_nric

INNER JOIN si_results e_2009_feen
    ON e_2009_feen.re_code LIKE 'FEEN%'
       AND e_2009_feen.re_year = '2009'
       AND e_2009_feen.re_semester = '2'
       AND e_2009_feen.re_nric = s.s_nric

INNER JOIN si_results e_2009_fema
    ON e_2009_fema.re_code LIKE 'FEMA%'
       AND e_2009_fema.re_year = '2009'
       AND e_2009_fema.re_semester = '2'
       AND e_2009_fema.re_nric = s.s_nric

LEFT JOIN si_isc_gpa AS isc
    ON isc.isc_g_nric = s.s_nric

WHERE  s.s_admission_year = '2008'
GROUP  BY s.s_nric
ORDER  BY s.s_gender,
          s.s_name ASC

编辑:包括标准化版本:

SELECT s.s_nric                           AS nric,
       s.s_name                           AS name,
       s.s_psle_eng                       AS psle_eng,
       s.s_psle_math                      AS psle_maths,
       s.s_psle_aggr                      AS psle_aggr,
       si_results.re_code                 AS code
       si_results.re_mark                 AS mark
       si_results.re_year                 AS year
       isc.isc_g_gpa                      AS isc_gpa

FROM   si_student_data AS s

INNER JOIN si_results e_2008_feen
    ON si_results.re_nric = s.s_nric

LEFT JOIN si_isc_gpa AS isc
    ON isc.isc_g_nric = s.s_nric

WHERE  s.s_admission_year = '2008'
    AND si_results.re_year in ('2008', '2009')
    AND si_results.re_semester = '2'
    AND (
          si_results.re_code LIKE 'FEEN%'
          OR si_results.re_code LIKE 'FEMA%'
     )

GROUP  BY s.s_nric
ORDER  BY s.s_gender,
          s.s_name ASC

这将产生如下行:

nric: 1
name: 'student 1'
psle_eng: eng1
psle_maths: maths1
psle_aggr: aggr1
code: FEENXXXX
mark: 5
year: 2008
isc_gpa: gpa1



nric: 1
name: 'student 1'
psle_eng: eng1
psle_maths: maths1
psle_aggr: aggr1
code: FEENXXXX
mark: 3
year: 2009
isc_gpa: gpa1


nric: 1
name: 'student 1'
psle_eng: eng1
psle_maths: maths1
psle_aggr: aggr1
code: FEMAXXXX
mark: 4.5
year: 2008
isc_gpa: gpa1


nric: 1
name: 'student 1'
psle_eng: eng1
psle_maths: maths1
psle_aggr: aggr1
code: FEMAXXXX
mark: 5
year: 2009
isc_gpa: gpa1

请注意,改变了四个记录的唯一值是code,mark和year.你将获得4倍的记录数量,但它的运行速度也应该比之前快得多.您的代码必须迭代所有行并根据需要聚合它们.