Chapter 11

Using Relational Operators


Bullet Combining tables with similar structures

Bullet Combining tables with different structures

Bullet Deriving meaningful data from multiple tables

You probably know by now that SQL is a query language for relational databases. In previous chapters, I present simple databases, and in most cases, my examples deal with only one table. In this chapter, I put the relational in “relational database.” After all, the name means “a database that consists of multiple related tables.” Here’s where you finally see those relationships.

Because the data in a relational database is distributed across multiple tables, a query usually draws data from more than one table. SQL has operators that combine data from multiple sources into a single result set. These are the UNION, INTERSECTION, and EXCEPT operators, as well as a family of JOIN operators. Each operator combines data from multiple tables in a different way.


The UNION operator is the SQL implementation of relational algebra's union operator. The UNION operator enables you to draw information from two or more tables that have the same structure. Same structure means

  • The tables must all have the ...

Get SQL For Dummies, 9th Edition now with the O’Reilly learning platform.

O’Reilly members experience live online training, plus books, videos, and digital content from nearly 200 publishers.