Coalesce Function is a useful function in oracle and it is very useful when we want to return first non null values in the list
expr1 : This is returned if it is not null
expr2: This is returned if it is not null and first expression is null
exprn: This is returned if the preceding expression are null
So Coalesce function will return first expression if it is not null else it will do the coalesce the rest of the expression
Important point
1)Oracle Database uses short-circuit evaluation. The database evaluates each expr value and determines whether it is NULL, rather than evaluating all of the expr values before determining whether any of them is NULL.
2)COALESCE is equivalent to Case function
So COALESCE (expr1, expr2)
is equivalent to:
CASE WHEN expr1 IS NOT NULL THEN expr1
ELSE expr2 END
Similarly,
COALESCE (expr1, expr2, …, exprn), for n>=3
is equivalent to:
CASE WHEN expr1 IS NOT NULL THEN expr1
WHEN expr2 IS NOT NULL THEN expr2
WHEN expr3 IS NOT NULL THEN expr3
WHEN exprn IS NOT NULL THEN exprn
END
Or this can be written as
CASE WHEN expr1 IS NOT NULL THEN expr1
ELSE COALESCE (expr2, …, exprn) END
3) Usually all the expr of the coalesce statement should be of the same datatype else ORA-00932 will be returned.
SQL> select coalesce(‘a’,1,2) from dual;
select coalesce(‘a’,1,2) from dual
*
ERROR at line 1:
ORA-00932: inconsistent datatypes: expected CHAR got NUMBER
If all occurrences of expr are numeric datatype or any nonnumeric datatype that can be implicitly converted to a numeric datatype, then Oracle Database determines the argument with the highest numeric precedence, implicitly converts the remaining arguments to that datatype, and returns that datatype.
Usage
SQL> select coalesce(1,2,3) from dual;
COALESCE(1,2,3)
—————
1
SQL> SELECT COALESCE(null, null, null, ‘X’, ‘Y’);
COALESCE(…)
—————
X
Suppose we have a employee table consisting address1,address2 and address3 and we can want to print the first not-null address of the employees from the table in order of priority address1 > address2 > address3
select emp_name, COALESCE(address1,address2,address3) from employee;