SAP HANA 利用case和MAP函数实现行转列

SAP HANA 利用case和MAP函数实现行转列：

范例表结构：

CREATE TABLE  SCORES(
 NAME NVARCHAR(30),
 SUBJECT NVARCHAR(30),
 SCORE   DECIMAL(5,2)
);

范例数据：

insert into SCORES values ('张三','语文',90.00);
insert into SCORES values ('张三','数学',91.00);
insert into SCORES values ('张三','英语',92.00);
insert into SCORES values ('李四','语文',93.00);
insert into SCORES values ('李四','数学',94.00);
insert into SCORES values ('李四','英语',95.00);
insert into SCORES values ('王五','语文',96.00);
insert into SCORES values ('王五','数学',97.00);
insert into SCORES values ('王五','英语',98.00);

 

使用case实现行转列：

 SELECT NAME,
            SUM(CASE WHEN SUBJECT='语文' THEN SCORE END) AS "语文",
            SUM(CASE WHEN SUBJECT='数学' THEN SCORE END) AS "数学",
            SUM(CASE WHEN SUBJECT='英语' THEN SCORE END) AS "英语"
   FROM SCORES
   GROUP BY NAME

使用MAP()函数实现行转列：

  
SELECT
   NAME,
   SUM(MAP(SUBJECT,'语文',SCORE,0)) AS "语文",
   SUM(MAP(SUBJECT,'数学',SCORE,0)) AS "数学",
   SUM(MAP(SUBJECT,'英语',SCORE,0)) AS "英语"
FROM SCORES
GROUP BY NAME;

 

MAP 函数：

官方文档：http://help.sap.com/hana/html/sql_function_map.html

语法：MAP (expression, search1, result1 [, search2, result2] ... [, default_result])

参数：MAP为键值对出现的数据结构，MAP函数也类似。expression为要寻找的键值，search1, result1 [, search2, result2] ... 定义相应的键值对，search1为键值，result1 为对应的结果。如果expression在键值对中找到对应的键值，则返回对应的结果，找不到对应的键值，如果有指定default_result值则返回default_result（为默认值），否则返回null

范例：

SELECT MAP(2, 0, 'Zero', 1, 'One', 2, 'Two', 3, 'Three', 'Default') "map" FROM DUMMY;

结果：map Two

SELECT MAP(99, 0, 'Zero', 1, 'One', 2, 'Two', 3, 'Three', 'Default') "map" FROM DUMMY;

结果：map Default

SELECT MAP(99, 0, 'Zero', 1, 'One', 2, 'Two', 3, 'Three') "map" FROM DUMMY;

结果：map NULL

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