SQL or Structured Query Language is a popular language for managing data in databases. It offers multiple tools for retrieving, changing, and analyzing data stored in tables. Joins and subqueries are two of SQL's most-used tools. Both of these tools can combine data from more than one table, but they do it in different ways. This blog will compare and contrast joins and subqueries and talk about how they are used in SQL.

What is a Join in SQL?

A join is a SQL operation that combines data from two or more tables based on a column that links them. The related columns are called "keys," and they are used to match the data in one table with the data in another table. A join is useful when it comes to combining data from multiple tables into a single set of results.

In SQL, one can do different kinds of joins, like left join, right join, inner join, and full outer join. Each type of join has its syntax and reason for being used.

Inner Join

An inner join only returns rows where the values in both tables are the same. It compares the values in the specified columns of both tables and puts the matching rows into a single result set.

Left Join

A left join gives all the rows from the left table and the rows from the right table that match them. If there are no matching rows in the right table, the result will have null values.

Right Join

Although the right join is similar to a left join, it returns all of the rows from the right table and the matching rows from the left table. If there are no rows in the left table that match, the result will have "null" values.

Full Outer Join

A full outer join returns all rows from both tables, even the ones that don't match. If there are no matching rows in one table, the result can contain null values for the columns in that table.

What is a Subquery in SQL?

A subquery is a query nested inside another query. Subquery is used as a way to retrieve data from one table based on the values of another table. A subquery is executed first and its result set is used in the main query as a condition to filter the data.

Subqueries may be used in different parts of a query, such as the SELECT, FROM, WHERE, and HAVING clauses. They can also be used in conjunction with other SQL clauses like EXISTS, NOT EXISTS, IN, NOT IN, ANY, ALL, and some aggregate functions.

Subqueries can be categorized into two types: correlated and non-correlated subqueries.

Non-Correlated Subqueries

A non-correlated subquery is a subquery that may be executed independently of the outer query. It is a query that retrieves data from one table and returns a single value, which is used in the main query as a condition to filter the data.

Correlated Subqueries

A correlated subquery is a subquery that is executed for each row of the outer query. After retrieving data from one table, it returns a set of values, which are used in the main query as a condition to filter the data.

Join vs Subquery

So, here's how joins and subqueries work. It is now time to let's compare them and see when to use each.

Joins are used to put together information from more than one table into a single set of results. They are useful when the data one requires is spread across multiple tables and needs to come together all in one query.

Subqueries, on the other hand, are used to filter data from a single table or to get data based on the results of another query. They help extract data that one can't otherwise get with a simple SELECT statement.

Most of the time joins are better than subqueries, especially when working with large datasets. Joins can also be easier to read and keep up with, especially when the condition needs to look at more than one table. But subqueries may be better in some cases, such as when the conditions are complicated or when one needs to get data from a single table based on the results of another query.

Here's a table summarizing the differences between joins and subqueries.

Conclusion

In conclusion, understanding the differences between joins and subqueries in SQL is crucial for any data professional. Joining tables can be more efficient in certain cases, while subqueries can provide more flexibility and control. Both techniques have their advantages and disadvantages, and it is up to the developer to decide which approach is best for their specific use case.

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