Your First Queries

Enough concepts — let us write SQL. The wonderful thing about PostgreSQL is that you can run a query without any tables at all, which makes it a perfect, friendly calculator for learning the shape of the language.

SQL you can run with no tables

The keyword SELECT means "compute and give me back these values." On its own, it can do arithmetic and produce text. Run this:

PostgreSQL evaluated 2 + 2 and handed back a one-row, one-column result containing 4. You just ran a complete, valid SQL query.

Naming results with AS

That last result column had an unhelpful auto-generated name. You can name a result column yourself with AS:

Notice three things that will recur all course long:

• You can SELECT several values at once, separated by commas.
• Text values go in single quotes: 'PostgreSQL'. Double quotes mean something different in SQL (they refer to names), so stick to single quotes for text.
• AS name labels each result column. The label is for you — it does not change any stored data.

The anatomy of a query

Here is the basic shape you will type thousands of times, with the parts named:

• SELECT — what you want back (columns or computed values).
• FROM — where to get it (which table).

When you are just computing values, like SELECT 2 + 2, you can skip FROM entirely. The moment you want stored data, you add FROM table_name.

Statements end with a semicolon

Each SQL statement ends with a semicolon ;. It marks where one instruction stops. With a single statement it is often optional, but it becomes essential when you run several statements together — so we will always include it as a good habit.

A first look at reading from a table

To prove the FROM half works, here is a tiny pre-loaded table. Reading from it is just SELECT columns FROM the table:

Try editing it: replace name, population with * to get every column, or with just country to get one. You are already steering a query — choosing exactly what comes back. The whole Querying Data section expands on this.

Check your understanding

QuestionSelect one

What does the SELECT keyword do in a query?

It permanently deletes the chosen columns.

It specifies which columns or computed values you want the database to return.

It creates a brand-new table.

It sorts the table alphabetically.

QuestionSelect one

In the query SELECT 5 * 3 AS total;, what is the role of AS total?

It multiplies the result by a value called total.

It stores the result permanently in a table named total.

It gives the resulting column the friendly name "total" in the output.

It is required or the query will fail.

QuestionSelect one

Which part of a query tells the database which table to read from?

SELECT

FROM

AS

The semicolon