Coalesce Function in Tsql

COALESCE Function in TSQL

I've been told that COALESCE is less costly than ISNULL, but research doesn't indicate that. ISNULL takes only two parameters, the field being evaluated for NULL, and the result you want if it is evaluated as NULL. COALESCE will take any number of parameters, and return the first value encountered that isn't NULL.

There's a much more thorough description of the details here
http://www.mssqltips.com/sqlservertip/2689/deciding-between-coalesce-and-isnull-in-sql-server/

Does SQL Server read all of a COALESCE function even if the first argument is not NULL?

Nope. Here's a simple test:

SELECT COALESCE(1, (SELECT 1/0)) -- runs fine
SELECT COALESCE(NULL, (SELECT 1/0)) -- throws error

If the second condition is evaluated, an exception is thrown for divide-by-zero.

Per the MSDN Documentation this is related to how COALESCE is viewed by the interpreter - it's just an easy way to write a CASE statement.

CASE is well known to be one of the only functions in SQL Server that (mostly) reliably short circuits.

There are some exceptions when comparing to scalar variables and aggregations as shown by Aaron Bertrand in another answer here (and this would apply both to CASE and COALESCE):

DECLARE @i INT = 1;
SELECT CASE WHEN @i = 1 THEN 1 ELSE MIN(1/0) END;

will generate a division by zero error.

This should be considered a bug, and as a rule COALESCE will parse from left to right.

What does a zero value coalesce function do?

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.

MSSQL COALESCE function with multiple non null values

I suspect what you want is something like the following, which uses a UNION instead of a JOIN.

SELECT a.t_stamp T_Stamp, a.Destination Destination, a.A_Weight Weight, 'A' Line
FROM Ham_Line_A_Counts a
WHERE a.t_Stamp Between '{Root Container.Popup Calendar.date}' AND '{Root Container.Popup Calendar 1.date}' AND a.Destination = {Root Container.Dropdown 1.selectedValue}
UNION ALL
SELECT b.t_stamp T_Stamp, b.Destination Destination, b.B_Weight Weight, 'B' Line
FROM b_Counts b
WHERE b.t_Stamp Between '{Root Container.Popup Calendar.date}' AND '{Root Container.Popup Calendar 1.date}' AND b.Destination = {Root Container.Dropdown 1.selectedValue})
ORDER BY T_Stamp DESC

This gets the results from a with the correct destination and timestamps, gets the results from b with the correct destination and timestamps, then sorts them all together by timestamp. So if a timestamp is in a and b, both rows are returned one after the other. I used UNION ALL rather than just UNION since the hardcoded 'A'/'B' in the Line column means there won't be duplicates and the UNION ALL could be more efficient.

What is the difference between IFNULL and COALESCE in MySQL?

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'

In what scenario the second/third non-null parameters will be returned in COALESCE mysql?

Coalesce returns first non-null value in the list of arguments, if the first argument is null. These list of arguments could be constant value or derivation of other column value.

Primarily used for scenarios where you have to derive value from some other column or replace null with value for column where null is not an accepted for a given column.

Example : If a column is representing discount% then the value could be either a value or 0.0 but cannot be null.

coalesce(special_discount,base_discount_column,0.05) 

Here special discount if null consider base discount value , if base discount is also null, then take 5% discount by default.

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