SQL Queries for Mere Mortals: A Hands-On Guide to Data Manipulation in SQL, Second Edition

Subqueries as Column Expressions

In Chapter 5, Getting More Than Simple Columns, you learned a lot about using expressions to generate calculated columns to be output by your query. We didn’t tell you then that you can also use a special type of SELECT statement—a subquery—to fetch data from another table, even if the table isn’t in your FROM clause.

Syntax

Let’s go back to the basics and take a look at a simple form of a SELECT statement in Figure 11-1.

Figure 11-1. The syntax diagram for a simple SELECT statement

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This looks simple, but it really isn’t! In fact, the value expression part can be quite complex. Figure ...