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String manipulation is the process for transforming existing data into another form that can be used by businesses to generate reports. SQL server has various built-in functions for manipulating and transforming data. On the other hand, it is important examining datasets, exploring data values, encode or decode values as required to generate more meaningful data.
It is also important to know how to navigate through missing values in datasets and understand the impact on queries, calculations, data-set generation, reports and come up with innovative techniques to avoid letting NULL values ruin our result sets.
Before we dive deep into the topic, let us first understand the NULL Values and their impact on datasets. It is necessary to have a clear idea of Null Values before you start learning to Coalesce function in SQL Server.
Null is a special marker used in Structured Query Language to check either a particular value exists or not in a database. This concept was introduced by the relational database model creator E.F. Codd. SQL Null values serve to fulfill the requirement that all true relational database management systems support a representation of missing and inapplicable values.
Codd used the omega symbol to represent NULL in database theory. In SQL, NULL is defined as the reserved keyword to identify this marker. Null should not be misunderstood with a zero (0) value. Null indicates the lack of value, and it could not equal to zero. A lack of answer does not mean that you should write “no” there.
Further, SQL NULL is not a value but a state. Its usage is quite different from other programming languages where the NULL value of a reference means it is not pointing to any object. SQL offers some handy functionality to work with the character data in SQL queries that we will be discussing in detail below.
The Coalesce function in SQL server is used to define the Null values. During the expression evaluation process, Null values are generally replaced with user-defined values. The function evaluates arguments in a specific order and always returns non-null values first from the given arguments list. The basic syntax of Coalesce function in SQL server is given below.
COALESCE (expression [1…n])
Here are some properties of SQL Coalesce function:
SELECT COALESCE (NULL,'A','B') SELECT COALESCE (NULL,100,20,30,40) SELECT COALESCE (NULL,NULL,20,NULL,NULL) SELECT COALESCE (NULL,NULL,NULL,NULL,NULL,'Prashanth') SELECT COALESCE (NULL,NULL,NULL,NULL,1,'Prashanth') SELECT COALESCE (NULL,NULL,NULL,NULL,NULL,'Prashanth',1)
Consider an example, you want to concatenate two strings together but what happens if the string has Null values. Let us move ahead and execute the T-SQL. As you parse through the string, you realized that there is a Null value in the string while processing string concatenation operation. SQL simply returns Null when it encounters the Null values. The sample syntax for string concatenation is given as below.
SELECT firstName +' '+MiddleName+' '+ LastName FullName FROM Person.Person
So, how to handle these Null values. The best idea is using coalesce function allows handling behavior of Null values gracefully. In this example, Null values for the middle name will be replaced by space once the query is executed. The SQL query still concatenates three fields, first name, middle name, and the last name but no Null values are shown in the output. The full name will display with a space between first name and the last name. Here is how to customize the same query using coalesce function in SQL Server.
SELECT firstName +' '+COALESCE(MiddleName,'') +' '+ LastName FROM Person.Person
The final output will look like this:
This example returns the concatenated non-null values from the table state. In some cases, there is a need for assigning concatenated static values to a variable. Here, you should use Coalesce function and concatenate within a single quote to prepare a string of values. Now, you should manipulate the output further to remove the last character and fetch the valid string in the end.
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)
The output of this query will look like this:
A user-defined function is used to return a string specific to the given input and output is grouped together using the group clause. For this example, the scalar-valued functions return the concatenated string values separated by commas for specified inputs. In this example, state values are grouped, city values are concatenated, and separated by a delimiter for the output. You may use String-AGG function too if using SQL Server 2017.
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
The output for this query will look like this:
SELECT state, city = dbo.tfn_CoalesceConcat(state) FROM dbo.state GROUP BY state ORDER BY state;
In this section, we will try to find out the emergency employee contacts. For most organizations, the phone number of employees is listed under work, home, mobile number columns. So, h
ow to find employees whose emergency contacts are not added in the list. Or you have to find out the list of employees with emergency numbers. Here is the sample program for data validation using Coalesce function in the SQL Server.
DROP TABLE IF EXISTS tb_EmergencyContact; CREATE TABLE tb_EmergencyContact ( empid int, firstname VARCHAR(100) NOT NULL, lastname VARCHAR(100) NOT NULL, relationship VARCHAR(100), homephone VARCHAR(25), workphone VARCHAR(25), cellphone VARCHAR(25) ); INSERT INTO tb_EmergencyContact ( empid, firstname, lastname, relationship, homephone, workphone, cellphone ) VALUES ( 1, 'Ambika', 'Prashanth', 'Wife', NULL, '920.176.1456', '928.132.2967' ),( 2, 'Prashanth', 'Jayaram', 'spouse', NULL, NULL, '982.132.2867' ), ( 3, 'Pravitha', 'Prashanth', 'Daughter', NULL, NULL, NULL )
Here, the coalesce function is used to return columns with phone numbers, home, work, etc. and Null values are returned if emergency contacts are not added for a particular employee.
SELECT firstname+''+lastname fullname, relationship, COALESCE(homephone, workphone, cellphone, 'NA') phone FROM dbo.tb_EmergencyContact
The coalesce function can be used to compare values of hourly wages, salary, commission, etc. and function return the Null values for the columns if found any.
CREATE TABLE EMP (EMPNO INT NOT NULL, ENAME VARCHAR(20), JOB VARCHAR(10), MGR INT, JOINDATE DATETIME, HOURLYWAGE DECIMAL(7,2), SALARY DECIMAL(7, 2), COMMISSION DECIMAL(7, 2), NUMSALES DECIMAL(7,2), DNO INT) INSERT INTO EMP VALUES (7369, 'SMITH', 'CLERK', 7902, '02-MAR-1970',NULL, 8000, NULL, 20,2), (7499, 'ALLEN', 'SALESMAN', 7698, '20-MAR-1971',NULL, 1600, 3000,4, 3), (7521, 'WARD', 'SALESMAN', 7698, '07-FEB-1983', 40,1250, 5000,10, 3);
Here is the list of total salary paid to all employees:
SELECT EMPNO,ENAME,CAST(COALESCE(HOURLYWAGE * 40 * 52, salary, Salary+(COMMISSION * NUMSALES)) AS decimal(10,2)) AS TotalSalary FROM dbo.EMP ORDER BY TotalSalary;
Moving ahead, let us see how to create a computed column with SQL Coalesce function in SQL Server. Generally, you should use the expressions in tables. In excel, it is required to compute values using several existing columns and a few scalar values within a table. Mostly these values are dependent on one or more columns. In this way, we can create a computed column using Coalesce function where Null Values can be managed even more efficiently.
ALTER TABLE dbo.EMP ADD Total_Salary AS CAST(COALESCE(HOURLYWAGE * 40 * 52, salary, Salary+(COMMISSION * NUMSALES)) AS decimal(10,2)) select * from EMP
To represent SQL, coalesce function syntactically, we can use the CASE expression. The Case expression in SQL first returns the Non-Null values then it managed the Null values for the program effectively. The above Coalesce statement in SQL using Case expression can be rewritten as:
This query will also return the same output as earlier:
The coalesce expression is a syntactic shortcut of Case expression in SQL. The basic syntax of coalesce function in SQL server is written as:
SELECT firstname+''+lastname fullname, relationship, CASE WHEN homephone is NOT NULL Then homephone WHEN cellphone is NOT NULL Then cellphone WHEN workphone is NOT NULL Then workphone ELSE 'NA' END EmergencyContactNumber FROM dbo.tb_EmergencyContact
The same syntax using CASE expression can be rewritten as:
In such cases, input values are evaluated multiple times. If there is one value expression that contains a subquery and considered non-deterministic, then the subquery is evaluated twice. In either case, the output value is calculated by working on first and upcoming evaluations.
Both functions are designed with the same objective, but they behave differently in certain situations as given below.
This blog explains tips and tricks on using SQL coalesce function effectively with the T-SQL. Coalesce function usually appears in a very specific content like query, view, stored procedure, etc. The usage of Coalesce function can be generalized by placing it within a function. You can also optimize its performance and make the results constantly available by placing it in a computed column.
We tried to include maximum information in the blog for your reference to use Coalesce function in different contexts. To know more on Coalesce function and similar SQL concepts, you may join SQL certification program and start your database learning now.
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