Detailed explanation of concat related functions in MySQL

1. concat() function

Function: Concatenate multiple strings into one string

Syntax: concat(str1,str2,…) The string can be either a data table field or a specified string.

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

2. concat_ws() function

Function: Like concat(), it concatenates multiple strings into one string, but you can specify the separator at one time (concat_ws is concat with separator)

Syntax: concat_ws(separator,str1,str2,…)

Note: The first parameter specifies the separator. It should be noted that the separator cannot be null. If it is null, all returned results will be null.

3. group_concat() function

Let’s look at this requirement first:

Where user_id is the user id, fee is the consumption amount, and time is the consumption timestamp.

In the consumption record table, you need to find out the details of all consumption amounts for each user.

Obviously, a single group by is not enough to meet this requirement:

select max(time),fee from consumption group by user_id

The relationship between the fields after group by is staggered. We can only filter a certain field, but cannot guarantee that they belong to the same record.

At this time, the group_concat() function is used:

select user_id,GROUP_CONCAT(time,':',fee) from consumption group by user_id

The following results are obtained:

By processing the results, we can easily display the desired effect on the page.

Summarize

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