Mysql case语句返回多行多列

Question

I have a table

    -id   --col_1 -- col_2 -- col3 --col_4 -- col5
    ------------------------------------------------
     1       a         b       c       d       e
     2       a1        b1      c1      d1      e1
     3       a2        b2      c2      d2      e2

I need to check if col3 contains a value, if present i need to display col1 and col2 values and remaining values should be empty. Another condition if col4 contains value, i need to display only col2 and col5 values

    -id   --col_1 -- col_2 -- col3 --col_4 -- col5
    ------------------------------------------------
     1       a         b       c              
     1                 b1             d1      e1
     2       a1        b1      c1     
     2                 b1             d1      e1
     3       a2        b2      c2       
     3                 b2             d2      e2

I am unaware of which procedure to follow to achieve my requirement. I have used case statement, i have retrieved only one record for two conditions with only one column

SELECT    
CASE
    WHEN col3 != '' THEN col4        
END as a, 
CASE
    WHEN col4 != '' THEN col2
END as b
FROM table;

Any Help!..

Answer 1

If I understand correctly, you just want union all :

select id, col1, col2, col3, NULL as col4, NULL as col5
from table
union all
select id, NULL, col2, NULL, col4, NULL as col5
from table ;