Man today home group_concat, was feeling some here know little, learn a lot.
A: concat basis functions
1. function usage
Syntax: concat (str1, str2, ...)
Connecting a plurality of strings into a single string.
2. Example
select concat(contract_id, ' - ', bd_code, ' - ', bd_name) as tmp from t_plm_contract_monitor;
effect:
It can be seen here is the data in each row, all the fields, be connected.
3. Notes
If a field is null, then return to this line of argument is a null [online].
experimenting:
turn out:
This line is not empty null, the test of true knowledge, no place is not displayed.
I do not know version of the problem. This mysql version is 5.6.
Two: concat_ws
1. Description
The above wording feeling a little tired, because each link breaks should write, is there a simple way?
How this function is as follows:
concat_ws(separator, str1, str2, ...)
As long as the first position, specify the delimiter will not be a problem.
2. Test
select concat_ws(' - ',contract_id,bd_code,bd_name) as tmp from t_plm_contract_monitor;
effect:
3. Description
Feeling of use is very good use.
4. less data would happen then?
Test results are as follows, it is still not displayed.
Three: group_concat
1. Description
This is today's main event.
2. Examples
By grouping contract id, find the smallest id.
select contract_id, min(id) from t_plm_contract_monitor group by contract_id;
result:
3. more and more demand
The above problem is relatively simple, but I want to know a contract_id corresponding id, what it does, and as multi-line contract_id data does not appear.
This time you can use this function a.
Example 4
select contract_id, GROUP_CONCAT(id) from t_plm_contract_monitor group by contract_id;
result:
The more optimized usage
Why do you say so, because the above usage, there is no explanation of how to use the delimiter.
GROUP_CONCAT (Field [order by sort field asc / desc] [distinct] connected to [Separator 'delimiter'])
6. to test
Let id be flashbacks, using the '-' segmentation.
select contract_id, GROUP_CONCAT(distinct id order by id desc separator '-') from t_plm_contract_monitor group by contract_id;
result:
7. Also one use
select group_concat(mm.contract_id) from ( select contract_id from t_plm_contract_bill_term_monitor where is_deleted = 0 GROUP by bill_term_end_date,contract_id HAVING count(*) > 1 ) mm
result:
