How can I change this query in so that return true if the select(*) count value is 2?

Question

I am not so into SQL and I have the following doubt. I have this query (I am using MySql):

SELECT count(*)                                AS exist
FROM MeteoForecast                             AS MF                         
WHERE 
     MF.localization_id = 22
AND
     DATE(MF.start_date) = DATE('2019/03/02') 

returning the number of record into the MeteoForecast table as the exist field.

I want to modify it in this way:

if the value of the count(*) is equal to 2, it returns true (the exist field is true), otherwise the exist field value is false.

How can I implement this behavior?

Answer 1

You can just use a boolean expression:

SELECT (count(*) = 2) AS exist2
FROM MeteoForecast mf                       
WHERE MF.localization_id = 22 AND
      DATE(MF.start_date) = DATE('2019-03-02') ;

This expression actually returns true as "1" and false as "0". Those are the numeric equivalents of the booleans true and false in MySQL.

Assuming that start_date has a time component, you should write this as:

SELECT (count(*) = 2) AS exist2
FROM MeteoForecast mf                       
WHERE MF.localization_id = 22 AND
      MF.start_date >= '2019-03-02' AND
      MF.stara_date < '2019-03-03' ;

This can take advantage of an index on MeteoForecast(localization_id, start_date), so it can be much faster.