Left Outer Join in SQL

Understanding the steps for extracting data from many tables is vital. It is recommended practice to adhere to the normalization rules in relational database systems, which essentially include dividing huge tables into smaller ones. Joins allow us to access the data from these tables in a select query. A left outer join is a method to merge tables that contain unmatched rows from the table established before the left outer join in SQL clause. The left join will return all rows from the left table and will match the rows from the right table. If there's no match, NULL values will be returned for the right table's columns.

What Is a LEFT OUTER JOIN?

A left outer join in SQL combines tables that include all the records from the first table in the join and any matching rows from the right table. The left outer join clause includes unmatched rows from only the table specified before the clause. The dominant table of the outer join appears to the left of the keyword that begins the outer join. In SQL, a left outer join, also known as a LEFT OUTER JOIN, retrieves all records from the left table (table1) and the matching records from the right table (table2). If no matching records are found in the right table, NULL values are present for those columns.

A left outer join returns the following result set:

• All rows that satisfy the join condition
• Rows from the left table that do not meet the join condition
• Matched records from the right table
  

Use case of left outer join

Some left outer join in SQL use cases are as follows:

• When all items from the left table should be displayed, even if there are no matching entries in the right table.
• Addressing incomplete or missing data: In circumstances where information in one of the tables may be lacking or incorrect.
• Optional relationships are used in situations when you wish to show data from the main table but do not require relationships between tables.
• Combining data: When you wish to keep all the records from one table while merging data from other tables.
• Retrieving all rows from the leftmost table: When you need to retrieve all rows from the leftmost table, even if no matching rows exist in the other table.
• Finding disconnected rows in tables involves identifying rows that do not have corresponding foreign keys in another table.
  

What’s the Distinction Between LEFT OUTER JOIN And LEFT JOIN?

A LEFT OUTER JOIN and a LEFT JOIN are essentially the same things. In SQL, the two variants of the syntax will provide the same outcome. Although it's completely optional, some experts do advise adding outer in a left outer join in SQL clause to make it obvious that you're creating an outer join.

The syntax for left outer join in SQL is:

SELECT column_name
FROM table1

LEFT OUTER JOIN table2 ON table1. column_name=table2;

LEFT [OUTER] JOIN Syntax

Here's a left outer join in SQL example:

SELECT Consumers.ConsumerName, Orders. Product 2 FROM Consumers 3 LEFT JOIN Orders 4 ON Consumers. ConsumerID = Orders. ConsumerID;

This would list all customers and their respective orders, but customers without orders would still be listed with a NULL for their order. A left outer join is essentially the same operation as a left join, and the keyword "OUTER" is optional in most database systems.

Importance of left outer join

A left outer join in SQL is used to combine the tables. The result will include only the unmatched rows from the table defined before the left outer join clause. So, if you are using the left outer join clause on two tables and want to include an unmatched row in your table, then simply apply the left outer join clause or the right outer join clause based on the join condition. The usage of this kind of JOIN is the same as that of LEFT JOIN. It also displays all of the information from the left table and only the rows from the right table that match. It will display NULL if no matching records are in the appropriate table. This left outer join will include the rows with no matching values in the table, which were specified after the left after join.

What is Left Join?

A left join is also known as a left outer join; it is an SQL operation that is used to retrieve all the information from one table and from another table, and it is joined using the left join operation. It retrieves all the common values from the two tables, and for the unmatched values, it assigns a null value to those values. The example of the left join can be:

When you wish to display every row from the left table and just the rows from the right table that match, you use this kind of JOIN. You are selecting information from the correct table. It will be indicated as NULL if any rows in the left table have values absent from the right table.

Syntax

SELECT column_name(s)
FROM table_name1
LEFT JOIN table_name2
ON table_name1.column_name = table_name2.column_name

Benefits

• Left joins are handy when combining data from several databases because they only include rows with identical values in the leftmost table.
• Even in cases where there are no matching rows in the other table, this kind of join can help collect all rows from the leftmost table.
• One table's records can be listed from one table or just those that cross over into another.

Limitations

• Execution of left join left outer join can be sluggish, particularly when dealing with big leftmost tables.
• Performance problems may arise if the linked tables have an excessive number of columns.
• A significant reduction in query performance may result from improper indexing of the columns in the joined tables.
• Interpreting the result set of a left join can be challenging.
  

Improving the Performance of Your Left Join Related SQL Statements

Left join left outer join is an effective method for retrieving data. However, working with big datasets or intricate queries may cause performance problems. You should think about optimizing your Left Join queries in the following ways to make sure they execute as fast and effectively as possible:

• Indexes: Ensure that the columns utilized in the join condition are indexed. This can significantly reduce query execution time and speed up the search procedure. Furthermore, it's critical to maintain any current indexes by confirming that they appropriately represent modifications made to database tables or columns.
• Limit Columns: Choosing which columns to include in a result set is crucial. Retrieving columns more often than necessary can increase resource usage and slower query execution.

Which Is Better to Use—Left Outer or Left Join?

However, we advise using the whole name—LEFT OUTER JOIN—if you're a novice SQL user. You can specify that your join is an outer join in this way. When you return to your code, the word OUTER will already be there, saving you time and effort. In this manner, you can effectively practice differentiating between an inner and outer join and knowing which to utilize until you master joins. If you're an advanced user, you'll likely find it useful to write less code when it can be done. Use a left join and left outer join in this situation.

Conclusion

A left outer join in SQL combines tables that include all records from the left table and unmatched rows from the right table. It's similar to an inner join but includes the remaining rows from one of the tables.

Frequently Asked Questions (FAQs)

1. What is left exclusive join in SQL?

A left exclusive join in SQL is mainly used to get the values existing only from the left table, i.e., you request the values exclusively from the left table without including any values from the right table.

2. What is left to join in SQL?

The left join command in SQL retrieves all the values present in the left table; only the matching values from the right table and the columns that don’t match will be given a null value.

3. What are the four types of JOINs in SQL?

The four kinds of join in SQL are:

• Inner join
• Left Outer Join
• Right Outer Join
• Self join
  

4. What is left join and right join?

The left outer join and the right outer join are both used to join the two tables together and retrieve the common values from both tables. The left join is used to retrieve the values from the right table and only common values from the left table, whereas the left join is used to retrieve values from the left table and only the common values from the right table.

5. What is a left outer join?

A left join or the left outer join is used to join two tables(right table and left table) together and helps us to make a query for all the details from the left table and only take the common values from the right table.

6. What is the difference between left outer join and left join?

The left outer join and left join have no difference; they both generate the same output.

7. How is the left outer join symbol?

The symbol of the left outer join is:

Left outer join (⟕)

8. IS LEFT join inner or outer?

The inner join returns values that are common in both tables only, whereas an outer join will return separate values for both the left table and the right table, depending on which outer join is used, either the left outer join or the right outer join.