【實戰分享】相同組獲取第一條資料的幾種SQL實現

阿新 • • 發佈：2020-12-30

技術標籤：sql db java sql db 資料庫 java 相同組獲取第一條資料

【實戰分享】相同組獲取第一條資料的幾種SQL實現

分組排序後獲取第一條資料
分組加行號後獲取第一條資料（當然獲取前幾條資料也是同理）

利用GROUP BY後聚合函式，比如：MAX, MIN等

此種方式簡單粗暴，存在一個隱藏問題，就是非分組列獲取值不一定是同一行的資料，比如：這裡的workout_time
此種方式適用於MariaDB，如果想適用於MySQL需要修改ANY_VALUE(wr.workout_date)，存在非分組列相同問題

SELECT       
  h.id AS 'hospital_id' 
, h.name AS 'hospital_name', p.id AS 'patient_id', p.name AS 'patient_name',       
  ed.game_id, g.name AS 'game_name', MAX(ed.score) AS 'game_score', wr.workout_date AS 'workout_time'     
FROM       
  t_p p,       
  t_w_r wr,      
  t_e_d ed,       
  t_h h,      
  t_g g      
WHERE  
  p.name = 
 'kenny' AND
  p.id = wr.patient_id      
  AND wr.id = ed.workout_id       
  AND p.hospital_id = h.id       
  AND ed.game_id = g.id       
  GROUP BY p.id, p.name, ed.game_id, workout_time      
  ORDER BY game_id, game_score DESC;

ROW_NUMBER() OVER(PARTITION BY [fields] ORDER BY [fields]) AS ‘row_num’

此種方式適用於MariaDB，不適用在低版本MySql，聽說8.0以上支援

SELECT * from (
SELECT ROW_NUMBER() OVER(PARTITION BY p.id, p.name, ed.game_id ORDER BY game_id, game_score DESC) AS 'row_num',
  h.id AS 'hospital_id', h.name AS 'hospital_name', p.id AS 'patient_id', p.name AS 'patient_name', p.gender,          
  ed.game_id, g.name AS 'game_name', g.limb, ed.score AS 'game_score', wr.workout_date AS 'workout_time'     
FROM       
  t_p p,       
  t_w_r wr,      
  t_e_d ed,       
  t_h h,      
  t_g g      
WHERE  
  -- p.name = 'kenny' and
  ed.game_id = 28 and
  workout_date >= '2019-09-11' and
  p.id = wr.patient_id   
  AND wr.id = ed.workout_id       
  AND p.hospital_id = h.id       
  AND ed.game_id = g.id            
  ORDER BY game_id, game_score DESC
) t
WHERE row_num = 1

SET @row_number = 0; SELECT @row_number:=IF(@pre_game_id = game_id, @row_number + 1, 1) AS 'row_num', @pre_game_id:=game_id AS 'game_id2', t.* FROM WHERE row_num = 1

此種方式適用於MySQL，但是，在MariaDB中使用需要解決一個問題，明明子查詢已經排序再設定行號，但是就是設定行號的時候不是按照排序後的，可以說行號設定在排序前進行了，因此出來的結果同一個分組同時存在幾條row_num = 1的情況
經試驗後，確定MariaDB需要加入GROUP BY方可分組排序後才能正確新增行號
SET @row_number = 0; 另外，這句不加，如果在Client第一次執行會出現行號都是1的情況
經試驗後，是因為@pre_game_id沒有初始值，而@row_number可以沒有初始值

SET @row_number = 0;

SELECT * FROM (
SELECT @row_number:=IF(@pre_game_id = game_id, @row_number + 1, 1) AS 'row_num', t.*, @pre_game_id:=game_id AS 'game_id2' FROM (
SELECT 
  h.id AS 'hospital_id', h.name AS 'hospital_name', p.id AS 'patient_id', p.name AS 'patient_name', p.gender,          
  ed.game_id, g.name AS 'game_name', g.limb, ed.score AS 'game_score', wr.workout_date AS 'workout_time'     
FROM       
  t_p p,       
  t_w_r wr,      
  t_e_d ed,       
  t_h h,      
  t_g g      
WHERE  
  p.name = 'kenny' AND
  workout_date >= '2010-09-11' AND
  p.id = wr.patient_id   
  AND wr.id = ed.workout_id       
  AND p.hospital_id = h.id       
  AND ed.game_id = g.id 
  -- MySQL 不需要此 GROUP BY，加了會報錯 this is incompatible with sql_mode=only_full_group_by
  GROUP BY game_id, game_score, workout_time       
  ORDER BY game_id, game_score DESC, workout_time DESC
) t
) m
WHERE 
  row_num = 1
  -- MariaDB 不需要此 ORDER BY，加了也沒關係
  ORDER BY game_id