SELECT ename,comm
FROM emp
WHERE ISNULL(comm,0)= 0;
Please explain this query .
This Query selects the emp records whose commission is null as well as zero.
ISNULL in MYSQL have only one parameter and it's true/false test if variable is null.
IFNULL in MYSQL has 2 params
IFNULL(expr1, expr2) - if expr1 is not NULL, IFNULL() returns expr1, otherwise it returns expr2.
IFNULL(expr1,0) return 0 in two cases:
expr1 = 0
expr1 is NULL (in that case second param - 0 is returned)
MYSQL IFNULL documentation
MYSQL DB Fiddle
In Transact-SQL there is ISNULL function with two params ISNULL ( check_expression , replacement_value ) and it works exactly like MYSQL IFNULL
T-SQL ISNULL documentation
SQL server DB Fiddle
COALESCE is an SQL function that returns the first non-NULL expression among its arguments. So in the following statement...
SELECT USER.user_id,
USER.firstname,
USER.lastname,
...
COALESCE(EMPLOYEE.title, '') title,
...
FROM USER
... it is basically saying that if EMPLOYEE.title is NULL, then return and use '' instead. Is my understanding correct?
Let's say that EMPLOYEE.title equals 'CEO'. If we plug this into the COALESCE function, our query would look something like:
SELECT COALESCE('CEO', '') sub_sector;
If we ran it, we would get 'CEO'. Now let's say that EMPLOYEE.title is NULL. If we plug that into the COALESCE function, our query would look something like:
SELECT COALESCE(NULL, '') sub_sector;
If we run that, we will get '' since COALESCE returns the first non-null value in its argument list. Since the first value is NULL, it will then check the next value, '', which is not NULL, so it will return it.
In the case of your query, if the field EMPLOYEE.title has a NULL value, the COALESCE function will return ''.
I have a WHERE CLAUSE that looks like this:
t.triggered > (SELECT MAX(time_flag) FROM triggers)
When the subquery returns a value because there is one the whole query is executed normally.
When the subquery has no returned value the comparison is always false and no records are returned even though everything is greater than 'NULL'.
I read that 'NULL' is not appropriate for comparisons, so how can the query be written in order to overcome this issue?
Something like:
t.triggered > COALESCE((SELECT MAX(time_flag) FROM triggers), '1900-01-01')
A predicate containing a NULL value, doesn't evaluate to either true, or false: it evaluates to NULL. Using COALESCE you can compare to a very old value in case the subquery returns NULL and hence return everything from your table.
Replace NULL value with default value (some minimal date) in subquery using IFNULL or COALESCE functions:
t.triggered > (SELECT IFNULL(MAX(time_flag), '1000-01-01') FROM triggers)
SELECT IFNULL(NULL, 'Replaces the NULL')
--> Replaces the NULL
SELECT COALESCE(NULL, NULL, 'Replaces the NULL')
--> Replaces the NULL
In both clauses the main difference is argument passing. For IFNULL it's two parameters and for COALESCE it's multiple parameters. So except that, do we have any other difference between these two?
And how it differs in MS SQL?
The main difference between the two is that IFNULL function takes two arguments and returns the first one if it's not NULL or the second if the first one is NULL.
COALESCE function can take two or more parameters and returns the first non-NULL parameter, or NULL if all parameters are null, for example:
SELECT IFNULL('some value', 'some other value');
-> returns 'some value'
SELECT IFNULL(NULL,'some other value');
-> returns 'some other value'
SELECT COALESCE(NULL, 'some other value');
-> returns 'some other value' - equivalent of the IFNULL function
SELECT COALESCE(NULL, 'some value', 'some other value');
-> returns 'some value'
SELECT COALESCE(NULL, NULL, NULL, NULL, 'first non-null value');
-> returns 'first non-null value'
UPDATE: MSSQL does stricter type and parameter checking. Further, it doesn't have IFNULL function but instead ISNULL function, which needs to know the types of the arguments. Therefore:
SELECT ISNULL(NULL, NULL);
-> results in an error
SELECT ISNULL(NULL, CAST(NULL as VARCHAR));
-> returns NULL
Also COALESCE function in MSSQL requires at least one parameter to be non-null, therefore:
SELECT COALESCE(NULL, NULL, NULL, NULL, NULL);
-> results in an error
SELECT COALESCE(NULL, NULL, NULL, NULL, 'first non-null value');
-> returns 'first non-null value'
Pros of COALESCE
COALESCE is SQL-standard function.
While IFNULL is MySQL-specific and its equivalent in MSSQL (ISNULL) is MSSQL-specific.
COALESCE can work with two or more arguments (in fact, it can work with a single argument, but is pretty useless in this case: COALESCE(a)≡a).
While MySQL's IFNULL and MSSQL's ISNULL are limited versions of COALESCE that can work with two arguments only.
Cons of COALESCE
Per Transact SQL documentation, COALESCE is just a syntax sugar for CASE and can evaluate its arguments more that once. In more detail: COALESCE(a1, a2, …, aN)≡CASE WHEN (a1 IS NOT NULL) THEN a1 WHEN (a2 IS NOT NULL) THEN a2 ELSE aN END. This greatly reduces the usefulness of COALESCE in MSSQL.
On the other hand, ISNULL in MSSQL is a normal function and never evaluates its arguments more than once. COALESCE in MySQL and PostgreSQL neither evaluates its arguments more than once.
At this point of time, I don't know how exactly SQL-standards define COALESCE.
As we see from previous point, actual implementations in RDBMS vary: some (e.g. MSSQL) make COALESCE to evaluate its arguments more than once, some (e.g. MySQL, PostgreSQL) — don't.
c-treeACE, which claims it's COALESCE implementation is SQL-92 compatible, says: "This function is not allowed in a GROUP BY clause. Arguments to this function cannot be query expressions." I don't know whether these restrictions are really within SQL-standard; most actual implementations of COALESCE (e.g. MySQL, PostgreSQL) don't have such restrictions. IFNULL/ISNULL, as normal functions, don't have such restrictions either.
Resume
Unless you face specific restrictions of COALESCE in specific RDBMS, I'd recommend to always use COALESCE as more standard and more generic.
The exceptions are:
Long-calculated expressions or expressions with side effects in MSSQL (as, per documentation, COALESCE(expr1, …) may evaluate expr1 twice).
Usage within GROUP BY or with query expressions in c-treeACE.
Etc.
Differences in SQL-Server:
There is no IFNULL() function but a similar ISNULL()
ISNULL takes only 2 parameters whereas COALESCE takes variable number of parameters
COALESCE is based on the ANSI SQL standard whereas ISNULL is a proprietary TSQL function
Validations for ISNULL and COALESCE is also different. For example, NULL value for ISNULL is converted to int, whereas for COAELSCE you have to provide a type. Ex:
ISNULL(NULL,NULL) : is int.
COALESCE(NULL,NULL) : will throw an error.
COALESCE(CAST(NULL as int),NULL) : is valid and returns int.
Data type determination of the resulting expression – ISNULL uses the first parameter type, COALESCE follows the CASE expression rules and returns type of value with highest precedence.
ifnull can only replace a null value of the first parameter. Whereas coalesce can replace any value with another value. With coalesce in standard SQL you can have many parameters transforming many values.
EDIT the example according to comments below.
Example: coalesce(null, null, null, 'b*', null, 'null*')
returns 'b*' and it is not possible to do with ifnull.
This db2 SQL will not work with COALESE, I will not see any rows retrieved.
Since I used IFNULL it is working as expected
select a.mbitno ,a.mbstqt,ifnull(b.apr,0)
from
(
select mmstcd,mbstat,mbfaci,mbwhlo,mbitno,mbstqt,MBALQT from libl.mitbal inner join libl.mitmas on
mmcono=mbcono and mmitno=mbitno
where mbcono=200 and mbstat in ('20','50') and mmstcd>0
)
as a left join
(
select mlfaci,mlwhlo,mlitno,mlstas,sum(mlstqt) as APR from libl.mitloc where mlcono=200 and mlstas='2'
group by mlfaci,mlwhlo,mlitno,mlstas
)
b on b.mlfaci=a.mbfaci and b.mlwhlo=a.mbwhlo and b.mlitno=a.mbitno
where a.mbitno in 'GWF0240XPEC' and a.mbstqt>0 and a.mbstqt<>ifnull(b.apr,0)
I am getting NULL values in the results of an operation in MySQL.
Is there a way to convert the NULL values into the value 0?
Yes, by using COALESCE.
SELECT COALESCE(null_column, 0) AS null_column FROM whatever;
COALESCE goes through the list of values you give it, and returns the first non-null value.
I am adding this answer because no one mentioned IFNULL function
You can use IFNULL
SELECT IFNULL(column_name, 0) FROM table_name;
IFNULL will return column's value (if it has something other than NULL) otherwise second parameter passed (in this case 0).
If you messed up and have NULLs in existing table layout and want zeros, here is solution:
UPDATE `table` SET `somefield`=0 WHERE `somefield` is null
There is the COALESCE method which return the first non-null parameter, in your case :
COALESCE(field, 0)
But you can use this if you want more :
COALESCE(field1, field2, 0)
MySQL:
SELECT COALESCE(Mycolumn, 0);
you can put the 0 value into your insert input method by casting it:(int)0