MySQL多条选择语句

Question

SET @row_number:=0;
SET @db_names:='';
SELECT @row_number:=CASE WHEN @db_names=app_id THEN @row_number+1 ELSE 1 END AS row_number,@db_names:=app_id AS db_names
FROM master_order 
ORDER BY app_id ;

When I run this statement it gives me result.

I want it like

select *from (SET @row_number:=0;
SET @db_names:='';
SELECT @row_number:=CASE WHEN @db_names=app_id THEN @row_number+1 ELSE 1 END AS row_number,@db_names:=app_id AS db_names
FROM master_order 
ORDER BY app_id ) as temp

where row_number = 1;

I want to select the first row of duplicate columns

Answer 1

You can simplify this like this:

SELECT * FROM master_order
ORDER BY app_id = '' DESC, app_id
LIMIT 1;

But for clarification on how it would've worked if this simplification would not be possible, either this way:

SET @row_number:=0;
SET @db_names:='';

select *from (
    SELECT @row_number:=CASE WHEN @db_names=app_id THEN @row_number+1 ELSE 1 END AS row_number,@db_names:=app_id AS db_names
    FROM master_order 
    ORDER BY app_id 
) as temp
where row_number = 1;

or this:

select *from (
    SELECT @row_number:=CASE WHEN @db_names=app_id THEN @row_number+1 ELSE 1 END AS row_number,@db_names:=app_id AS db_names
    FROM master_order 
    CROSS JOIN (SELECT @row_number := 0, @db_names := '') var_init
    ORDER BY app_id 
) as temp
where row_number = 1;