Select PostgreSQL 中的条件替换

Question

My table has end_date column as Date type

It will have an actual value, a default value or NULL value

• If end date is populated for an employee, it will have actual date
• Default date is 2000-01-01 in yyyy-MM-dd format which is populated on 1st of every month
• When a new employee joins in the middle, it has NULL value

I need to make a query which will return the difference in days if it has an actual end date or blank if NULL or default value is there.

Select GREATEST(0, DATE_PART('day', end_date::timestamp - CURRENT_DATE))

This is the best bet I could manage as of now. Any way of doing it without If-Else conditions?

Answer 1

Try to use this in your select-list:

NULLIF(end_date, '2000-01-01') - CURRENT_DATE

The NULLIF if will return NULL , iff both parameters are the same, and the first parameter otherwise. Subtraction with a NULL value (on any side of the operator) will yield NULL as well, thus this will give you a "blank" output if end_date is not set or is set to the default (if you desire some other special value, surround this "query" with a COALESCE and add your default-blank-value as its second parameter). Otherwise you get your desired difference.

I do not see, why there should be any need to cast to TIMESTAMP . This will make the subtration to return an INTERVAL , but you want an INTEGER and this is what you get by subtracting two DATE s.