Understanding Coalesce in SQL From Scratch

Using the SQL Coalesce Function in SQL Server

The built-in character functions in the SQL Server allow you to manipulate and transform data. To produce meaningful data, it is also necessary to analyze the dataset, explore data values, and encode or decode the values as required. It's critical to understand how to work with missing values in the datasets, how they affect calculations, queries, reports, and data-set planning, and how to avoid letting null values ruin your result sets.

To manage NULL values, the Coalesce in SQL  and IsNull functions are used. The user-defined values replace the NULL values during the expression evaluation process.

The Coalesce in SQL  function evaluates the arguments in the specified order and always returns the first non-null value from the argument list.

SYNTAX:  COALESCE ( expression [1..n] )

What is a NULL value?

Null is a condition in Coalesce in SQL, not a value. This contrasts with most programming languages, where a null value for a reference indicates that it does not point to any entity.

Coalesce in SQL has some valuable features for dealing with character data in SQL queries, which you will go through in-depth.

In Structured Query Language, Null (or NULL) is a unique marker that indicates that a data attribute does not exist in the database. SQL Null was created by E. F. Codd, the inventor of the relational database model, to meet the requirement that all accurate relational database management systems (RDBMS) accept a representation of "missing information and inapplicable information." NULL is a reserved word in SQL that is used to define this marker.

SQL Coalesce Function

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.

Properties of the SQL Coalesce function and examples

• The data types of the expressions must be the same
• It can have multiple expressions in it
• Coalesce in SQL  is a syntactic shortcut for the Case expression in SQL
• An integer is always evaluated first, and an integer followed by a character expression produces an integer as an output

Example 1:

SELECT COALESCE (NULL,'Shobha','Shivakumar')

Output: 

Example 2:

SELECT COALESCE (NULL,'Shobha', 10, 'Shivakumar')

Output: 

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Example 3:

SELECT COALESCE (NULL, 10, 'Shobha', 'Shivakumar')

Output: 

Example 4:

SELECT COALESCE (NULL, 10, 20, 30)

Output: 

Example 5:

SELECT COALESCE (NULL, 20, 10, 30)

Output: 

Example 6:

SELECT COALESCE (NULL, 20, NULL)

Output: 

SQL Coalesce in a String Concatenation Operation

In the following example, you will concatenate a few values. However, this is again just a recollection of what happens when you have a NULL sense. Now, begin this with the T-SQL. And you can see that during the string concatenation process, it finds a NULL value. If Coalesce finds a NULL value in a SQL Server, it only returns NULL. It doesn't produce a combination of first, null, or last names.

SQL SELECT firstName +' '+MiddleName+' '+ LastName FullName FROM Person.Person

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Coalesce Pivoting 

You need to use the example below to locate connections with emergency workers. Telephone numbers for employees normally appear in each company's work, home, and cell phone columns.

You need to see how to find employees who have no emergency connections or, in other words, how to obtain all the details about an emergency employee.

DROP TABLE IF EXISTS STATE;

CREATE TABLE STATE

(

CITY VARCHAR(50),

STATE VARCHAR(500))

INSERT INTO STATE VALUES

('Appleton','WI'),('Milwaukee','WI'),('Madison','WI'),('Miami','Florida'),('Jacksonville','Florida')

DECLARE @col nvarchar(MAX);

SELECT @col = COALESCE(@col,'') +''''+CITY +''''+ ',' 

FROM dbo.STATE WHERE state = 'WI';

SELECT substring(@col,1,len(@col)-1)

Output

Scalar User-Defined Function and SQL Coalesce Function

expression1 

A value of any built-in or user-defined data form is returned by this expression.

expression2 

This function returns a value that corresponds to the data type in expression 1 of any built-in or user-defined data type. The section rules for results data types explain that the data types are mutually compatible.

The arguments are assessed to be established, and the outcome of the function is the first argument to be non-null. The result is zero if all claims are zero.

Data Validation Using SQL Coalesce Function

This group key is used to group output and to return a string unique to the given input with a user-specified function. For given inputs in this example, the scalar-valued functions return combined string values separated by commas. State values are collected, town values associated and in this case, a delimiter divides the contribution. You can also use the String-AGG function if you use SQL Server 2017.

Example 

CREATE FUNCTION dbo.tfn_CoalesceConcat

(

  @state varchar(100)

)

RETURNS NVARCHAR(MAX)

AS

BEGIN

  DECLARE @str NVARCHAR(MAX); 

  SELECT @str = COALESCE(@str + ', ', '') + CITY

FROM dbo.STATE

 WHERE state = @state

 ORDER BY state; 

  RETURN (@str);

END

GO

Output

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Conclusion

This blog explains effective tips and tricks on using Coalesce in SQL  features or Coalesce SQL Oracle effectively with T-SQL. Coalesce feature typically occurs in certain content like query, view, stored procedure, etc. The Coalesce in SQL can be used in a function to make it more general. By putting it in a computed column, you can improve its accuracy and make the results visible at all times.

This article provides plenty of details to use the Coalesce feature in various situations. Join theSQL Certification Course program from Simplilearn and begin your database learning right now to learn more about the Coalesce in SQL  and other SQL concepts.

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