A row subquery is a subquery returning a single row, as opposed to a scalar
subquery, which returns a single column from a row, or a literal.

Examples
--------

CREATE TABLE staff (name VARCHAR(10), age TINYINT);

CREATE TABLE customer (name VARCHAR(10), age TINYINT);

INSERT INTO staff VALUES ('Bilhah',37), ('Valerius',61), ('Maia',25);

INSERT INTO customer VALUES ('Thanasis',48), ('Valerius',61), ('Brion',51);

SELECT * FROM staff WHERE (name,age) = (SELECT name,age FROM customer WHERE
name='Valerius');
+----------+------+
| name     | age  |
+----------+------+
| Valerius |   61 |
+----------+------+

Finding all rows in one table also in another:

SELECT name,age FROM staff WHERE (name,age) IN (SELECT name,age FROM customer);
+----------+------+
| name     | age  |
+----------+------+
| Valerius |   61 |
+----------+------+

URL: https://mariadb.com/kb/en/subqueries-row-subqueries/