Concatenate date and varchar

Question

I am need to select the date stored in a column transactionDate datatype date and time stored in column transactionTime datatype varchar from a table transactionTable in mysql. My query is like

select concat( CAST( transactionDate as char), transactionTime) as transaction  
from transactionTable 
where id = 123

There are values in both the columns but the above returns me a null. How can I concat date and varchar columns?

Answer 1

The CONCAT() function returns NULL when one of the arguments is null. You could use COALESCE() to bypass the issue, converting the nulls to empty strings - or to any other constant value, for example '00:00:00' for the time part:

select concat( coalesce(cast(transactionDate as char), ''), 
               ' ', 
               coalesce(transactionTime, '00:00:00')
             ) as transaction  
from transactionTable 
where id = 123 ;

In fact, you don't really need an explicit cast(), the cast to char is implicit:

select concat( coalesce(transactionDate, ''), 
               ' ', 
               coalesce(transactionTime, '00:00:00')
             ) as transaction  
from transactionTable 
where id = 123 ;

Answer 2

You can also use CONCAT_WS function which (i) converts datatype automatically (ii) skips NULL values*:

SELECT CONCAT_WS(' ', transactionDate, transactionTime) AS whatever

^{* If transactionDate is NULL then the result won't contain leading space. If transactionTime is NULL then result won't contain trailing space.}