Understanding SQL LEFT JOIN with Examples

SQL LEFT JOIN

Multiple tables can be combined into a single result set in SQL using the JOIN method, which is based on a relevant column. Among the various types of joins, the LEFT JOIN is particularly useful when you want to retrieve all records from one table while fetching matching records from another table where available.

Only the matching records from the right table are included when an LEFT JOIN is performed; all data from the left table are returned. In the event that no match is discovered, NULL will appear in the right table’s columns.

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SQL LEFT JOIN Syntax

The general syntax for performing a LEFT JOIN operation in SQL is:

SELECT Table1.Column1, Table2.Column2
FROM Table1
LEFT JOIN Table2
ON Table1.CommonColumn = Table2.CommonColumn;

Let’s now take a closer look into LEFT JOIN using actual MySQL database samples.

Example 1: Employee and Department Tables

Consider two tables: staff and unit.

Staff Table

Unit Table

Query to Perform LEFT JOIN:

SELECT s.StaffID, s.Staff_Name, s.Salary, u.UnitID, u.Unit_Name
FROM staff s
LEFT JOIN unit u
ON s.StaffID = u.Staff_ID;

Output:

Example 2: Loans and Customers

Consider two tables: credit and client.

Credit Table

Client Table

Query to Perform LEFT JOIN:

SELECT c.LoanID, c.Branch, c.Amount, cl.ClientID, cl.Client_Name
FROM credit c
LEFT JOIN client cl
ON c.LoanID = cl.LoanID;

Output:

Example 3: Customers and Orders

Consider two tables: buyers and purchases.

Buyers Table

Purchases Table

Query to Perform LEFT JOIN:

SELECT b.BuyerID, b.Buyer_Name, b.Age, b.Salary, p.PurchaseID, p.Purchase_Date, p.Amount
FROM buyers b
LEFT JOIN purchases p
ON b.BuyerID = p.BuyerID;

Output:

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By using LEFT JOIN, you can efficiently combine data from multiple tables while ensuring that all records from the left table are included, even if no corresponding data exists in the right table.

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