Are there non obvious differences between NVL and Coalesce in Oracle?
The obvious differences are that coalesce will return the first non null item in its parameter list whereas nvl only takes two parameters and returns the first if it is not null, otherwise it returns the second.
It seems that NVL may just be a 'Base Case" version of coalesce.
Am I missing something?
NVL will do an implicit conversion to the datatype of the first parameter, so the following does not error
select nvl('a',sysdate) from dual;
COALESCE expects consistent datatypes.
select coalesce('a',sysdate) from dual;
will throw a 'inconsistent datatype error'
NVL and COALESCE are used to achieve the same functionality of providing a default value in case the column returns a NULL.
The differences are:
Examples for the third case. Other cases are simple.
select nvl('abc',10) from dual; would work as NVL will do an implicit conversion of numeric 10 to string.
select coalesce('abc',10) from dual; will fail with Error - inconsistent datatypes: expected CHAR got NUMBER
Example for UNION use-case
SELECT COALESCE(a, sysdate)
from (select null as a from dual
union
select null as a from dual
);
fails with ORA-00932: inconsistent datatypes: expected CHAR got DATE
SELECT NVL(a, sysdate)
from (select null as a from dual
union
select null as a from dual
) ;
succeeds.
More information : http://www.plsqlinformation.com/2016/04/difference-between-nvl-and-coalesce-in-oracle.html
Actually I cannot agree to each statement.
"COALESCE expects all arguments to be of same datatype."
This is wrong, see below. Arguments can be different data types, that is also documented: If all occurrences of expr are numeric data type or any nonnumeric data type that can be implicitly converted to a numeric data type, then Oracle Database determines the argument with the highest numeric precedence, implicitly converts the remaining arguments to that data type, and returns that data type.. Actually this is even in contradiction to common expression "COALESCE stops at first occurrence of a non-Null value", otherwise test case No. 4 should not raise an error.
Also according to test case No. 5 COALESCE does an implicit conversion of arguments.
DECLARE
int_val INTEGER := 1;
string_val VARCHAR2(10) := 'foo';
BEGIN
BEGIN
DBMS_OUTPUT.PUT_LINE( '1. NVL(int_val,string_val) -> '|| NVL(int_val,string_val) );
EXCEPTION WHEN OTHERS THEN DBMS_OUTPUT.PUT_LINE('1. NVL(int_val,string_val) -> '||SQLERRM );
END;
BEGIN
DBMS_OUTPUT.PUT_LINE( '2. NVL(string_val, int_val) -> '|| NVL(string_val, int_val) );
EXCEPTION WHEN OTHERS THEN DBMS_OUTPUT.PUT_LINE('2. NVL(string_val, int_val) -> '||SQLERRM );
END;
BEGIN
DBMS_OUTPUT.PUT_LINE( '3. COALESCE(int_val,string_val) -> '|| COALESCE(int_val,string_val) );
EXCEPTION WHEN OTHERS THEN DBMS_OUTPUT.PUT_LINE('3. COALESCE(int_val,string_val) -> '||SQLERRM );
END;
BEGIN
DBMS_OUTPUT.PUT_LINE( '4. COALESCE(string_val, int_val) -> '|| COALESCE(string_val, int_val) );
EXCEPTION WHEN OTHERS THEN DBMS_OUTPUT.PUT_LINE('4. COALESCE(string_val, int_val) -> '||SQLERRM );
END;
DBMS_OUTPUT.PUT_LINE( '5. COALESCE(SYSDATE,SYSTIMESTAMP) -> '|| COALESCE(SYSDATE,SYSTIMESTAMP) );
END;
Output:
1. NVL(int_val,string_val) -> ORA-06502: PL/SQL: numeric or value error: character to number conversion error
2. NVL(string_val, int_val) -> foo
3. COALESCE(int_val,string_val) -> 1
4. COALESCE(string_val, int_val) -> ORA-06502: PL/SQL: numeric or value error: character to number conversion error
5. COALESCE(SYSDATE,SYSTIMESTAMP) -> 2016-11-30 09:55:55.000000 +1:0 --> This is a TIMESTAMP value, not a DATE value!
Another proof that coalesce() does not stop evaluation with the first non-null value:
SELECT COALESCE(1, my_sequence.nextval) AS answer FROM dual;
Run this, then check my_sequence.currval;
NVL: Replace the null with value.
COALESCE: Return the first non-null expression from expression list.
Table: PRICE_LIST
+----------------+-----------+
| Purchase_Price | Min_Price |
+----------------+-----------+
| 10 | null |
| 20 | |
| 50 | 30 |
| 100 | 80 |
| null | null |
+----------------+-----------+
Below is the example of
[1] Set sales price with adding 10% profit to all products.
[2] If there is no purchase list price, then the sale price is the minimum price. For clearance sale.
[3] If there is no minimum price also, then set the sale price as default price "50".
SELECT
Purchase_Price,
Min_Price,
NVL(Purchase_Price + (Purchase_Price * 0.10), Min_Price) AS NVL_Sales_Price,
COALESCE(Purchase_Price + (Purchase_Price * 0.10), Min_Price,50) AS Coalesce_Sales_Price
FROM
Price_List
Explain with real life practical example.
+----------------+-----------+-----------------+----------------------+
| Purchase_Price | Min_Price | NVL_Sales_Price | Coalesce_Sales_Price |
+----------------+-----------+-----------------+----------------------+
| 10 | null | 11 | 11 |
| null | 20 | 20 | 20 |
| 50 | 30 | 55 | 55 |
| 100 | 80 | 110 | 110 |
| null | null | null | 50 |
+----------------+-----------+-----------------+----------------------+
You can see that with NVL we can achieve rules [1],[2]
But with COALSECE we can achieve all three rules.
Though this one is obvious, and even mentioned in a way put up by Tom who asked this question. But lets put up again.
NVL can have only 2 arguments. Coalesce may have more than 2.
select nvl('','',1) from dual; //Result: ORA-00909: invalid number of arguments
select coalesce('','','1') from dual; //Output: returns 1
There is also difference is in plan handling.
Oracle is able form an optimized plan with concatenation of branch filters when search contains comparison of nvl result with an indexed column.
create table tt(a, b) as
select level, mod(level,10)
from dual
connect by level<=1e4;
alter table tt add constraint ix_tt_a primary key(a);
create index ix_tt_b on tt(b);
explain plan for
select * from tt
where a=nvl(:1,a)
and b=:2;
explain plan for
select * from tt
where a=coalesce(:1,a)
and b=:2;
nvl:
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2 | 52 | 2 (0)| 00:00:01 |
| 1 | CONCATENATION | | | | | |
|* 2 | FILTER | | | | | |
|* 3 | TABLE ACCESS BY INDEX ROWID| TT | 1 | 26 | 1 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | IX_TT_B | 7 | | 1 (0)| 00:00:01 |
|* 5 | FILTER | | | | | |
|* 6 | TABLE ACCESS BY INDEX ROWID| TT | 1 | 26 | 1 (0)| 00:00:01 |
|* 7 | INDEX UNIQUE SCAN | IX_TT_A | 1 | | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(:1 IS NULL)
3 - filter("A" IS NOT NULL)
4 - access("B"=TO_NUMBER(:2))
5 - filter(:1 IS NOT NULL)
6 - filter("B"=TO_NUMBER(:2))
7 - access("A"=:1)
coalesce:
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 26 | 1 (0)| 00:00:01 |
|* 1 | TABLE ACCESS BY INDEX ROWID| TT | 1 | 26 | 1 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IX_TT_B | 40 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("A"=COALESCE(:1,"A"))
2 - access("B"=TO_NUMBER(:2))
Credits go to http://www.xt-r.com/2012/03/nvl-coalesce-concatenation.html.
Source: Stackoverflow.com