【SQL学习笔记】《SQL进阶教程》1.1

SQL进阶教程学习笔记1.1

1-1case 表达式

将已有编号方式转换为新的方式并统计

SELECT CASE pref_name
			WHEN '辽宁' THEN '东北'
			WHEN '福建' THEN '东南'
		ELSE '其他' END AS district,
		SUM(population)
	FROM PopTb1
	GROUP BY CASE pref_name
			WHEN '辽宁' THEN '东北'
			WHEN '福建' THEN '东南'
		ELSE '其他' AS END;

SELECT CASE pref_name
			WHEN '辽宁' THEN '东北'
			WHEN '福建' THEN '东南'
		ELSE '其他' END AS district,
		SUM(population)
FROM PopTb1
GROUP BY district;	

注意事项 END\ELSE

用一条语句进行不同条件的统计

SELECT pref_name,
	   --男性
	   SUM( CASE WHEN sex = "男" THEN population ELSE 0 END) AS cnt_m,
	   --女性
	   SUM( CASE WHEN sex = "女" THEN population ELSE 0 END) AS cnt_f
FROM PopTbl2
GROUP BY pref_name;

分别统计：将行结构转换成列结构。

SELECT 语句 条件分支

用CHECK约束定义多个列的条件关系

蕴含式逻辑 VS 逻辑“与”

在UPDATE中条件分支

--CASE + UPDATE
UPDATE Salaries
	SET salary = CASE WHEN salary >= 300000
					  THEN salary * 0.9
					  WHEN salary >= 250000 AND salary < 280000
					  THEN salary * 1.2
					  ELSE salary END;
					  --最后一行表示不符合以上条件的返回初始值

表之间的数据匹配

--use IN
SELECT course_name,
	   CASE WHEN course_id IN
	   				(SELECT course_id FROM OpenCourses
                      WHERE month = 200706) THEN 'O'
                      ELSE 'X' END AS "June",
       CASE WHEN course_id IN 
       				(SELECT course_id FROM OpenCourses
                      WHERE month = 200707) THEN 'O'
                      ELSE 'X' END AS "July"
FROM CourseMaster;

--use EXISTS better than IN, BECAUSE 性能更好
SELECT CM.course_name,
	   CASE WHEN EXISTS
       				(SELECT course_id FROM OpenCourses AS OC
                      WHERE month = 200706
                    	AND OC.course_id = CM.course_id) THEN 'O'
                 ELSE 'X' END AS "June",
	   CASE WHEN EXISTS
       				(SELECT course_id FROM OpenCourses AS OC
                      WHERE month = 200707
                    	AND OC.course_id = CM.course_id) THEN 'O'
                 ELSE 'X' END AS "July"
FROM CourseMaster AS CM;                 

case中使用聚合函数

--获取仅参加一个club的club_id、以及参加多个的main_club_id
SELECT std_id,
	   CASE WHEN COUNT(*) = 1 
	   		THEN MAX(club_id)
	   		ELSE MAX(CASE WHEN main_club_flg = 'Y'
	   					  THEN club_id
	   					  ELSE NULL END)
	   END AS main_club
FROM StudentClub
GROUP BY std_id;

CASE是一个表达式，而不是语句，可作为值进行返回。

练习题1-1-1 多列数据取最值

--两两比大小
SELECT key,
	   CASE WHEN x > y THEN x
	   ELSE y END AS greatest
FROM Greatests;	   

--三个排序
SELECT key,
       CASE WHEN CASE WHEN x < y THEN y ELSE x END < z
            THEN z
            ELSE CASE WHEN x < y THEN y ELSE x END
        END AS greatest
  FROM Greatests;

​ CASE WHEN CASE WHEN x < y THEN y ELSE x END < z

​ max{x, y} = CASE WHEN x < y THEN y ELSE x END

练习题1-1-2 转换行列——在表头里加入汇总和再揭

/* 转换行列——在表头里加入汇总和再揭(p.287) */
SELECT sex,
       SUM(population) AS total,
       SUM(CASE WHEN pref_name = '德岛' THEN population ELSE 0 END) AS col_1,
       SUM(CASE WHEN pref_name = '香川' THEN population ELSE 0 END) AS col_2,
       SUM(CASE WHEN pref_name = '爱媛' THEN population ELSE 0 END) AS col_3,
       SUM(CASE WHEN pref_name = '高知' THEN population ELSE 0 END) AS col_4,
       SUM(CASE WHEN pref_name IN ('德岛', '香川', '爱媛', '高知')
                THEN population ELSE 0 END) AS zaijie
  FROM PopTbl2
 GROUP BY sex;

练习题1-1-3 特定顺序

/* 用ORDER BY生成“排序”列 */
SELECT key
  FROM Greatests
 ORDER BY CASE key
            WHEN 'B' THEN 1
            WHEN 'A' THEN 2
            WHEN 'D' THEN 3
            WHEN 'C' THEN 4
            ELSE NULL END;

/* 把“排序”列也包括在结果中(p.288) */
SELECT key,
       CASE key
         WHEN 'B' THEN 1
         WHEN 'A' THEN 2
         WHEN 'D' THEN 3
         WHEN 'C' THEN 4
         ELSE NULL END AS sort_col
  FROM Greatests
 ORDER BY sort_col;