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A little help here. I really don't understand how to use this coalesce in MySQL
I have read all the pages in page 1 result of how to use coalsece in google result.
I know its meaning that it returns the first non-null value it encounters and null otherwise.
But it's still vague for me.
coalesce(column1,column2)? what if first column is null and other column is not null?
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The SQL COALESCE function can be syntactically represented using the CASE expression. For example, as we know, the Coalesce function returns the first non-NULL values. SELECT COALESCE (expression1, expression2, expression3) FROM TABLENAME; The above Coalesce SQL statement can be rewritten using the CASE statement.
The coalesce in MySQL can be used to return first not null value. If there are multiple columns, and all columns have NULL value then it returns NULL otherwise it will return first not null value. The syntax is as follows. SELECT COALESCE(yourColumnName1,yourColumnName2,yourColumnName3,.......
The SQL server's Coalesce function is used to handle the Null values. The null values are replaced with user-defined values during the expression evaluation process. This function evaluates arguments in a particular order from the provided arguments list and always returns the first non-null value.
Data type determination of the resulting expression is different. ISNULL uses the data type of the first parameter, COALESCE follows the CASE expression rules and returns the data type of value with the highest precedence.
How come i saw queries that returns multiple values? isnt it only the first not null value that is returned?
Yes, it is only the first non-NULL value that is returned. You must be mistaken about the queries you have seen where you thought that was not the case: if you could show us an example, we might be able to help clarify the misunderstanding.
And how do it decide which column to base? coalesce(column1,column2)? what if first column is null and other column is not null?
In order of its arguments: in this example, column1 before column2.
Or if im wrong or my syntax is wrong, how do i properly write it?
You're not wrong.
Can someone provide a very good and simple example on how to use it?
Taken from the documentation:
mysql> SELECT COALESCE(NULL,1); -> 1 mysql> SELECT COALESCE(NULL,NULL,NULL); -> NULL
And when it is desirable to use.
It is desirable to use whenever one wishes to select the first non-NULL value from a list.
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I personally use coalesce when I want to find the first column that isn't blank in a row from a priority list.
Say for example I want to get a phone number from a customer table and they have 3 columns for phone numbers named mobile, home and work, but I only want to retrieve the first number that isn't blank.
In this instance, I have the priority of mobile, then home and then work.
TABLE STRUCTURE -------------------------------------------- | id | customername | mobile | home | work | -------------------------------------------- | 1 | Joe | 123 | 456 | 789 | -------------------------------------------- | 2 | Jane | | 654 | 987 | -------------------------------------------- | 3 | John | | | 321 | -------------------------------------------- SELECT id, customername, COALESCE(mobile, home, work) AS phone FROM customers RESULT ------------------------------ | id | customername | phone | ------------------------------ | 1 | Joe | 123 | ------------------------------ | 2 | Jane | 654 | ------------------------------ | 3 | John | 321 | ------------------------------ If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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