Concat and concat_ Ws() differences and groups_ Use of concat() and repeat() functions

1 ：concat Function can connect one or more strings , Such as

select concat('10');//10
 
select concat('11','22','33');//112233

2：concat Function when connecting strings , As long as one of them is NULL, Then it will return to NULL

select concat('11','22',null);//null

3：concat_ws() function , 
Express concat with separator, That is, string connection with separator

select concat_ws(',','11','22','33');//11,22,33
 
select concat_ws('|','11','22','33');//11|22|33
 
select concat_ws('*','11','22',NULL);//11*22

and concat The difference is , concat_ws When the function is executing , Not because NULL Value NULL 

4：

group_concat()

Available row to column

The complete grammar is as follows :

group_concat([DISTINCT] Fields to connect [Order BY ASC/DESC Sort field ] [Separator ‘ Separator ’])

Example :

create table aa(
  id int,
  name VARCHAR(255)
 
);
 
insert  into aa values(1,10);
insert  into aa values(1,10);
insert  into aa values(1,20);
insert  into aa values(1,30);
insert  into aa values(3,30);
insert  into aa values(5,60);
insert  into aa values(5,90);
insert  into aa values(6,990);
 
1  With id grouping , hold name The value of the field is printed on one line , Comma separated ( Default )
 
select id,group_concat(name) from aa group by id;
 
2  With id grouping , hold name The value of the field is printed on one line , Semicolons separate 
 
select id,group_concat(name separator ';') from aa group by id;
 
3  With id grouping , Remove redundant name The value of the field is printed on one line , Comma separated 
 
select id,group_concat(distinct name separator ';') from aa group by id;
 
4  With id grouping , hold name The value of the field is printed on one line ,* No separation , With name Reverse order 
 
select id,group_concat(name order by name desc separator "*") from aa group by id;
 

eg:

select * from aa;

 select id,group_concat(name) from aa group by id;

select id,group_concat(name separator ';') from aa group by id;

select id,group_concat(distinct name separator ';') from aa group by id;

 

select id,group_concat(name order by name desc separator "*") from aa group by id;