intro
Subqueries make up an integral part of the work of every database developer. It is widely known that subqueries are just queries within queries, but they’re so much more than that. This article will take you through everything you need to know – this is worth checking out.
Subqueries are queries within queries. In other words, they‘re nested queries – one query is called the inner query, while another query (the one inside the inner query) is called an outer query. In the majority of the cases, here‘s how subqueries look like:
In other words, a subquery is a SELECT statement inside of another SELECT statement. Subqueries often are used to select data from another table, and they also start with the IN clause. The syntax for the majority of subqueries is as follows (colX is the column that is used for search results - where we run search queries on):
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SELECT t1.colA, t1.colB
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FROM table1 AS t1
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WHERE t1.colX IN (
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SELECT t2.colC, t2.colD
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FROM table2 AS t2
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WHERE t2.colY = 'Tutorial'
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);
Similar to the query above, subqueries are helpful when we wish to search for data in a more sophisticated manner than is often provided by our preferred database management system. Subquery inquiries typically:
Finally, databases are able to create "exotic" results—results that address more challenging issues than those answered by conventional queries—as a result of subqueries. Let us now look at some types of subqueries.
Types of Subqueries in PostgreSQL
There are types of subqueries in Postgres. The various types of subqueries are as follows:
For example, the subquery we’ve built above, (SELECT column3 FROM table2 WHERE condition), returns a single row and a single column value. The outer query then compares column2 from table1 with the value returned by the subquery.
In the example above, the subquery (SELECT column3 FROM table2 WHERE condition) returns multiple rows and a single column value. The outer query uses the IN operator to compare column2 from table1 with the set of values returned by the subquery.
In this example, the subquery (SELECT MAX(column2) FROM table2 t2 WHERE t2.column3 = t1.column3) is correlated to the outer query by referencing t1.column3 from the outer query within the subquery. For each row that is processed in the outer query, the subquery is executed, and the result is based on the specific value of t1.column3 for that row.
Distinctions of Subqueries
The query we’ve built above is supposed to return a row, if it does otherwise, the database management system with throw an error. Let’s note that the query below would throw an error too:
However, if we want to send a request like “find all rows in a tutorial table that overlap with the rows in a tutorial_2 table”, we can use subqueries as well. See the example below:
When to Use A Subquery
By now you should have gained an understanding of subqueries. But when exactly should we use a subquery? This is a very important question. Let’s look at some specific use cases:
Summary
In this blog, we’ve gone through the implications, upsides, and downsides of using subqueries in database management systems. We’ve told you what they are, how they work, and how best to utilize subqueries. We hope you’ve enjoyed reading this blog and that you’ll stay around our blog to learn more about databases in the future.
FAQs
What is a Subquery?
Subqueries are just queries within queries – subqueries or nested queries.
When do I use Subqueries?
Use a subquery when you need to select data and perform multiple operations on it at the same time. By way of explanation, use subqueries whenever the result that you wish to achieve requires you to use a subset of more than one table.
Can a subquery return multiple columns?
Yes. Subqueries can. This can be done by ensuring that the columns and their respective data types match their usage in the context of the subquery.
How do I use Subqueries?
To use subqueries is quite simple. Let’s say that a, b, x, d, e, and f are columns, a query like this would do:
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SELECT a,b from my_table
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WHERE x IN (SELECT d, e FROM my_table2 WHERE f = 'Tutorial');
