SQL subqueries are very interesting indeed and they accomplish the use of one query embedded in another query. These sub-queries are useful in performing further computations where the output of one sub-query is the input of the other. A subquery can be used more or less anywhere in the more encompassing SQL statement, for instance, in the SELECT, INSERT, UPDATE or DELETE clauses. Thus, through the inclusion of subqueries in your SQL queries, one can run several queries at one time thereby speeding up the retrieval and manipulation of data.

What is a Subquery?

In a simpler context, a subquery could be termed as a query embedded in another query. Subqueries are often known as inner queries while the queries that make use of subqueries are known as outer queries. Thus, with the use of sub-queries more complex queries can be formed since the results of the inner queries can be used in the outer ones.

Typically, the subquery value will either be a single value, a set of values or a table of values based on the manner and the location the subquery is implemented. It is written inside brackets, and placed in a position where an expression is needed. Thus, whenever an inner query is performed, it is done before the outer query and the result obtained forms the input of the outer query.

Types of Subqueries

We can group subqueries according to their placement and the number of values they return. The different types of sub-queries include the following:

1. Scalar Subquery:

This type of sub-query is the best because it returns only one value. Scalar subqueries are most frequently placed where a single value is required such as in SELECT statements or within the WHERE clause. The outcome of the upper query can be a computed element which entails an expression that does a comparison with the value from the scalar subquery.

2. Row Subquery:

This type of the row subquery is designed to retrieve one row which contains several columns of data. This row is needed in cases where multiple inner and outer row values are being compared.

3. Column Subquery:

These types of sub-queries return multiple rows but single column values within that set, thus a single value of many columns can be a sub-query. They are mostly used when there is the need to specify a specific column in the outer query and compare with the values returned from the sub-query set.

4. Table Subquery:

A table sub-query returns many columns and rows of information. It is suitable when the outer query requires perusal of a table that encompasses a set of rows and columns returned in the inner query.

The Places that Subqueries Can Be Utilized

The use of subqueries can be seen in different parts of a SQL statement. Most subqueries are used within the following places:

- SELECT Clause:

Subqueries in the SELECT clause allow for the computation of any values necessary for the results set of the query(s). For example, instead of calculating and placing the maximum or average of a group of data in the results of your regular query, a subquery could be used to carry out such functions.

- WHERE Clause:

This is the other area where most subqueries are used. A subquery in the WHERE clause filters certain values resulting from the main query with certain conditions provided by a secondary query. In that case, the result of the inner query is used to determine whether other columns in the main, or the outer query, meet certain conditions.

- FROM Clause:

A subquery in the FROM clause works like a derived table in which it provides a new temporary table for the main query to search from. This provides an opportunity to carry out more advanced joins or filters on the information that will be provided by the subquery.

- INSERT, UPDATE, DELETE Clauses:

Subqueries can also be included in data manipulation commands such as INSERT, UPDATE, DELETE by using it to provide values for insertion or to specify the records which need updating or deletion using the results of another query.

Advantages of Using Subqueries

Subqueries have a number of benefits, especially in the formulation of complicated SQL queries, including the following:

- Modularity:

Sub-queries help to subdivide complicated queries into simpler ones. Each sub-query acts as a part of the query and is comprehensible and interchangeable with other parts of the query.

- Efficiency:

In certain situations, a subquery will prove more effective than making a series of queries. The reason for this is that the subquery enables the user to perform a number of related queries in one operation.

- Flexibility:

Subqueries give you the ability to take the results of one query, and run another query using those results. This gives you the ability to deal with multi-step processes which would be hard to communicate in a single query.

Points to Note When Employing Subqueries

Subqueries are useful but as with everything else there are some things that one should keep in mind when employing them:

- Performance:

The performance of a query may be affected by subqueries especially if they are placed inside loops or when a lot of data is being brought back via the sub-query. In some cases, subqueries can be replaced by JOINs which in certain cases may perform better.

- Nested Subqueries:

SQL has the capability of allowing a subquery to be nested within another subquery. Although this is a nice feature, subqueries that are located too deep can be difficult to read. They also make it less appealing and less efficient to use subqueries.

- Readability:

Subqueries can also make a query difficult to read for example if they reside under multiple layers of nested subqueries. The overall clarity of the query can be made prominent by careful writing of the subqueries so that the main query can be easily understandable.

- Limitations:

The general picture is that not all types of sub-queries are supported by all the SQL database management systems (DBMS). It is advisable to know the weaknesses of the particular DBMS you are using and change your queries with respect to that particular DBMS.

Subqueries vs. Joins

One can say that subqueries and joins are the reverse of each other, as from two tables they create but a single one. It is always the case that subqueries are utilized to extract a single value or a set of values which are then employed in an outer query. When there exists a common column, joins are used to extract records from more than one table.

In some situations it might be worthwhile to use joins instead of subqueries in joins where combining rows in several stages improves efficiency or makes your query clearer. Unlike when working with filters or having to perform specific calculations based on other subqueries that you’re dealing with, then subqueries tend to be preferable.