MySQL COALESCE with two arguments? - mysql

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 ''.

Related

What does a zero value coalesce function do?

I have been exploring dbt tools and I came across the following code snippet :
coalesce(customer_orders.number_of_orders, 0) as number_of_orders
I understand that a coalesce function is used to return the first non-null value in a list. What I do not understand is what does the zero in the second parameter signify?

The COALESCE function returns the first non-null value in a list. COALESCE can take n number of arguments.
COALESCE(val1, val2, ...., val_n)
So according to the query:
coalesce(customer_orders.number_of_orders, 0) as number_of_orders
In case customer_orders.number_of_orders is NULL the result returned in number_of_orders would be 0.

COALESCE can use as many arguments as you want. In most cases (like in your example), COALESCE(some_column,0) is used to prevent that creating a sum or building an average will not lead to the desired result.
Assume there are three columns and you want to sum them. In case you don't use COALESCE, the sum will be NULLeven if only one of your three columns is NULL. So you will use COALESCEand replace NULL values by zero in order to receive the sum of all NOT NULL values.
You can "translate" COALESCE into a CASE WHEN construct:
COALESCE(column1,0)
does following:
CASE WHEN column1 IS NULL THEN 0 ELSE column1 END
Another use case of COALESCE is to replace column1 by column2 if column1 is NULL, if also column2 is NULL, take column3 etc.
I created an example here, so you can see what I mean:
db<>fiddle

MySql, CONCAT and NOT IN, string values

I have such query but it doesn't select anything, but it should. So query
SELECT *
FROM _custom_access_call
WHERE CONCAT(type, name) NOT IN ('string1', 'string2', 'string3')
I manually add to table entry with null and '1sfgsg' values but it wasn't selected. Why? I need to select all entries that concat values is not in array. Help to deal with it.

If one of the values is NULL, then CONCAT() will return NULL. And NULL NOT IN (...) is always NULL. Also NULL IN (...) is always NULL. If you want to use NULL you should explicitally handle it. In this specific case, CONCAT_WS() helps, because it never returns NULL.
SELECT *
FROM _custom_access_call
WHERE CONCAT_WS('', type, name) NOT IN ('string1', 'string2', 'string3');
Also, note that this query cannot use any index.

What is the difference between IFNULL and COALESCE in MySQL?

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)

sql check the select return null

I want to make an query can check completely null
for example:
select a where
case
when a like %b%
then a like %b% (if after search every row of table still return null)
else
a like %c% (if have match in any row skip else statement)

Sounds like you need the COALESCE() function:
SELECT COALESCE(a, b, c, ..., 'default')
this'll return the first NON-null value in the argument list, e.g.
COALESCE(null, null, 'hello')
returns hello.

Is there a function equivalent to the Oracle's NVL in MySQL?

I'm selecting the max of a column from a table. But there is one problem: if there are no rows in the table, it returns null.
I want to use a function which will return a certain value if the result is null. For example with Oracle there is the NVL function which gives a certain value if the column is null. Is there an equivalent function in MySQL ?

Use coalesce:
select coalesce(column_name, 'NULL VALUE') from the_table

or you can use IFNULL(expr1,expr2)
If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2.
select IFNULL(column_name, 'NULL VALUE') from the_table;
taken from:
https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html#function_ifnull