Analysis of concat and group in MySQL_ Use of concat

Hello everyone , I meet you again , I'm the king of the whole stack .

The examples used in this article are in the following database tables tt2 perform ：

One 、concat() function

1、 function ： Concatenate multiple strings into a single string .

2、 grammar ：concat(str1, str2,…)

The return result is the string generated by the connection parameter , If any of the parameters is null, The return value is null.

3、 give an example ：

example 1:

select concat (id, name, score) as info from tt2;

Copy

There is an act in the middle null Because tt2 There is a row in the table score The value is null.

example 2： In case 1 Three fields in the result of id,name,score The combination of does not have a delimiter , We can add a comma as a separator ：

This seems to be pleasing to the eye a lot ～～

But input sql Statements are a lot of trouble , You need to enter commas twice for the three fields , If 10 A field , To enter a comma nine times … The trouble is dead , Is there any easy way ？—— You can then specify the separator between parameters concat_ws() coming ！！！

Two 、concat_ws() function

1、 function ： and concat() equally , Concatenate multiple strings into a single string , But you can specify the separator all at once ～（concat_ws Namely concat with separator）

2、 grammar ：concat_ws(separator, str1, str2, …)

explain ： The first parameter specifies the separator . It should be noted that the separator cannot be null, If null, Then the returned result is null.

3、 give an example ：

example 3: We use concat_ws() take The separator is specified as a comma , Reach and example 2 Same effect ：

example 4： Specify the separator as null, It all turned out to be null：

3、 ... and 、group_concat() function

Preface ： There is group by In the query statement of ,select The specified field is either contained in the group by After statement , As a basis for grouping , Or in aggregate functions .

example 5：

This example queries name The smallest of the same people id. If we want to check name All of the same people id Well ？

Of course, we can query like this ：

example 6：

But the same name appears many times , It doesn't look intuitive . Is there a more intuitive way , Let each name appear only once , It can show all the people with the same name id Well ？—— Use group_concat()

1、 function ： take group by The resulting values in the same group are connected , Returns a string result .

2、 grammar ：group_concat( [distinct] Fields to connect [order by Sort field asc/desc ] [separator ‘ Separator ’] )

explain ： By using distinct You can exclude duplicate values ; If you want to sort the values in the results , have access to order by Clause ;separator Is a string value , The default is a comma .

3、 give an example ：

example 7： Use group_concat() and group by Show people with the same name id Number ：

example 8： The above id The numbers are sorted from large to small , And use ’_’ As a separator ：

example 9： The query above shows that name All in each group in the group id. Next, we will query to name Of all groups grouped id and score：

Publisher ： Full stack programmer stack length , Reprint please indicate the source ：https://javaforall.cn/111434.html Link to the original text ：https://javaforall.cn