SQL: Coalesce

Coalesce is a simple but useful SQL function I use quite often. It returns the first non-null value in a list of values.

A common real world application for this function is when you are trying to join data from multiple sources. If you work for a company known for acquiring other companies, your HR tables are bound to be full of all kinds of inconsistencies.

In this example below, you will see a snip-it of an HR table created by merging tables from two companies. The first two employees have an emp_id and the last one has a b_emp_id. The last one, Priyanka, is an employee from a newly acquired company, so her emp_id is from the HR table of the newly acquired company.

c1

So if you just want to merge the 2 emp_id columns into one, you can use the coalesce() function.

The syntax for coalesce is: coalesce (col1,col2,….)  as alias

Coalesce takes the first non-null. In the example below, we check emp_id first, if it has a value, we use that. If emp_id is null, we look for the value in b_emp_id.

select emp_nm,
coalesce (emp_id, b_emp_id) as emp_id
from employee_id

c2

If you want to try this for yourself, here is the code to build the table out for you.

CREATE TABLE employee_id (
        emp_nm nvarchar(30) not null,
             emp_id nvarchar(8),
             b_emp_id nvarchar(8)
             PRIMARY KEY(emp_nm) );
INSERT INTO employee_id
       (emp_nm, emp_id)
VALUES
       ('Bob', 'A1234567'),
       ('Lisa', 'A1234568')
INSERT INTO employee_id
       (emp_nm, b_emp_id)
VALUES
       ('Priyanka', 'B1234567');

 

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