Coalesce() function

sql Medium COALESCE() function
about Oracle database , Generally, the functions that often deal with null values are NVL, and mysql What is often used in ifnull, These two functions are similar , It's all derived from a function , That's it COALESCE() function .
COALESCE() function Definition ： Returns the first non in the list null Value of expression . If all expressions evaluate to null, Then return to null.
COALESCE() Functions have two uses ：

1.COALESCE ( expression1, expression2 );
2.COALESCE ( expression1, expression2, ... expression-n );

The first one is equivalent to Oracle Medium nvl perhaps mysql Medium ifnull, The form of the expression is ：

CASE WHEN expression1 IS NOT NULL THEN expression1 
ELSE expression2 
END;

The second can include n Expression , Indicates that if the first one is not empty, the first one will be taken , Otherwise, judge the next , And so on , If all are empty , Then return to null value .
Attention should be paid to ：vertica Empty string followed by null Values are not the same