In MySQL, the LEFT JOIN clause is used to combine rows from two or more tables based on a related column between them. The ON keyword is used to specify the conditions for joining the tables.
For example, consider two tables: “customers” and “orders”. Each customer can have multiple orders, so we can join these tables using the customer_id column:
SELECT *
FROM customers
LEFT JOIN orders
ON customers.customer_id = orders.customer_id;
This query will return all customers, including those who haven’t placed any orders yet. The matching rows from the “orders” table will be included in the result set if they exist, otherwise NULL values will be returned.
On the other hand, the LEFT function in MySQL is used to extract a substring from a given string starting from the left side of that string. For example:
SELECT LEFT('Hello World', 5);
This query will return ‘Hello’, which is the first five characters of the input string ‘Hello World’.
So, to summarize:
- Use LEFT JOIN when you want to join two or more tables based on a common column.
- Use LEFT function when you want to extract a substring from a string starting from its left side.
