拼接：concat、concat_ws和形成集合collect_set、collect_list用法

学习三个函数：
concat 进行字符串拼接
collect_set() 将分组的数据变成一个set集合。里面的元素是不可重复的。
collect_list(): 里面是可以重复的。
concat_ws(分隔符,集合) : 将集合中的所有元素通过分隔符变为字符串。

 实例：

需求：
-- 将下面的数据格式
zhangsan        chinese 90
zhangsan        math    87
zhangsan        english 63
zhangsan        nature  76
lisi    chinese 60
lisi    math    30
lisi    english 78
lisi    nature  0
wangwu  chinese 89
wangwu  math    25
wangwu  english 81
wangwu  nature  9
-- 转成：
zhangsan chinese:90,math:87,english:63,nature:76
lisi chinese:60,math:30,english:78,nature:0
wangwu chinese:89,math:25,english:81,nature:9

 （行转列）

 

select name,concat(subject,":",cj) from map_temp;

结果：
name    _c1
zhangsan        chinese:90
zhangsan        math:87
zhangsan        english:63
zhangsan        nature:76
lisi    chinese:60
lisi    math:30
lisi    english:78
lisi    nature:0
wangwu  chinese:89
wangwu  math:25

以上这个结果再合并：
select name,collect_set(concat(subject,":",cj)) from map_temp
group by name;

lisi    ["nature:0","english:78","math:30","chinese:60"]
wangwu  ["math:25","chinese:89"]
zhangsan        ["nature:76","english:63","math:87","chinese:90"]
将集合中的元素通过逗号进行拼接：
select name,concat_ws(",",collect_set(concat(subject,":",cj))) from map_temp group by name;

结果：
zhangsan chinese:90,math:87,english:63,nature:76
lisi chinese:60,math:30,english:78,nature:0
wangwu chinese:89,math:25,english:81,nature:9

 想将数据变为：

lisi    {"chinese":"60","math":"30","english":"78","nature":"0"}
wangwu  {"chinese":"89","math":"25"}
zhangsan        {"chinese":"90","math":"87","english":"63","nature":"76"}

 需求：将字符串变为map集合  使用一个函数 str_to_map

 select name,str_to_map(concat_ws(",",collect_set(concat(subject,":",cj)))) from map_temp group by name;