In this lesson, let's take a closer look at a subquery that returns one row and one column. This type of subquery is also known as a scalar subquery.
It can be used in various parts of the main SQL query, but most often it is used in selection restriction conditions using comparison operators (=, <>, >, <).
The following simplest query demonstrates the output of a single value (company name). In this form, it doesn't make much sense, but your queries can be much more complex.
SELECT (SELECT name FROM company LIMIT 1) AS company_name;
Similarly, scalar subqueries can be used to filter rows using WHERE, using comparison operators.
SELECT * FROM FamilyMembers WHERE birthday = (SELECT MAX(birthday) FROM FamilyMembers);
With this query, it is possible to obtain the youngest family member. The subquery in this case is necessary to obtain the maximum date of birth, which is then used in the main query to filter rows.
When using the result of a subquery with comparison operators, as in our example, it is important that the subquery returns exactly a scalar value (1 row and 1 column).
If this subquery returned multiple values, the DBMS would return an error, indicating that it expected the subquery to return only 1 record: «ER_SUBQUERY_NO_1_ROW: Subquery returns more than 1 row».
Therefore, it is important to be careful when writing subqueries and to understand what result the subquery will return, and what operators we can use together with the resulting set.