A question asked about data contained in two or more tables in a relational database. The relational query must specify the tables required and what the condition is that links them; for example, matching account numbers. Relational queries are tricky to specify because even the simplest of questions may require data from two or more tables. Both the knowledge of the query language and the database structure is necessary. Even with graphical interfaces that let you drag a line from one field to another, you still need to know how the tables were designed to be related.
An Early Relational Query
In the mid-1980s, Paradox was the first DBMS on a PC that made linking tables easier. Although not as sophisticated as today's methods, the ability to associate relationships by typing sample words was a breakthrough. The Customer No. and Part No. fields are related by "ABC" and "XYZ." Any words would suffice as long as they are the same.