SQL中的case when then else end用法
当我们需要从数据源上 直接判断数据显示代表的含义的时候 ,就可以在SQL语句中使用 Case When这个函数了.
Case具有两种格式。简单Case函数和Case搜索函数。
简单函数
CASE [col_name] WHEN [value1] THEN [result1]…ELSE [default] END
搜索函数
CASE WHEN [expr] THEN [result1]…ELSE [default] END
达梦数据库建表SQL
CREATE TABLE "TEST"."CJT" ( "ID" INT NOT NULL, "KC" VARCHAR(50), "FS" INT, "NAME" VARCHAR(50), CLUSTER PRIMARY KEY("ID")) STORAGE(ON "TEST", CLUSTERBTR) ; COMMENT ON TABLE "TEST"."CJT" IS '成绩表'; COMMENT ON COLUMN "TEST"."CJT"."ID" IS '主键'; COMMENT ON COLUMN "TEST"."CJT"."KC" IS '课程名称'; COMMENT ON COLUMN "TEST"."CJT"."FS" IS '分数'; COMMENT ON COLUMN "TEST"."CJT"."NAME" IS '姓名';
执行SQL
SELECT NAME AS "姓名", (CASE KC WHEN '语文' THEN FS ELSE 0 END) AS "语文", (CASE KC WHEN '数学' THEN FS ELSE 0 END) AS "数学", (CASE KC WHEN '英语' THEN FS ELSE 0 END) AS "英语" FROM TEST.CJT
执行SQL
SELECT NAME AS "姓名", MAX(CASE KC WHEN '语文' THEN FS ELSE 0 END) AS "语文", MAX(CASE KC WHEN '数学' THEN FS ELSE 0 END) AS "数学", MAX(CASE KC WHEN '英语' THEN FS ELSE 0 END) AS "英语" FROM TEST.CJT GROUP BY CJT.NAME;