mysql case when

MySQL in case when Usage of ：

select f1,case [f2] 

when expression1 then value1

when expression2 then value2

else value3

end alias_field

from table;

Equivalent to... In the code ：

if(expression1 ){
alias_field = value1;
}elseif(expression2 ){
alias_field = value2;
}else{
alias_field = value3;
}

case [f2] Express f2 This field is optional ,expression Is a conditional expression or constant ,value It's a value , stay expression and value You can use the value of the field for judgment or operation ;

For example, an activity data table , Each activity has a start time and an end time , During the presentation, it is required to rank the activities in progress first , Those that have not started are in the middle , The finished ones are at the bottom

Activity list ：activity

id Primary key

activity_name The name of the event

start_time Activity start time

end_time End time

Query activity data according to the above sorting rules ：

select id,activity_name,
case
                    when end_time < unix_timestamp(now())  then  -1   /* Has ended */
                    when start_time < unix_timestamp(now()) and end_time > unix_timestamp(now())  then  1     /* Have in hand */
                    else 0      /* Not yet begun */
                    end order_number
from activity order by order_number desc limit 1,10;