# SELECT Basics

> Select values and columns, name query results, and inspect system catalogs.

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LLMS index: [llms.txt](/en/llms.txt)

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`SELECT` is the heart of SQL — it is how you read data. The simplest form is `SELECT expression`, with no `FROM`, which evaluates the expression and returns one row. The more common form, `SELECT columns FROM table`, returns one row per matching row in the table. Either way, the result is a *relation*: rows and columns, with column names you can rename using `AS`.

## What you'll learn {#learn}

- `SELECT` with literals and expressions (no table needed)
- Naming result columns with `AS`
- Selecting multiple columns in one query
- Reading from a system catalog table
- Column aliases vs. table aliases

## Example {#example}

```sql {num="3.1" caption="SELECT Basics"}
-- Literals and expressions: no FROM required
SELECT 1 AS one;
SELECT 2 + 3 AS sum, 10 / 4 AS quotient, 10 / 4.0 AS real_quotient;
SELECT 'hello' AS greeting, length('hello') AS chars;

-- Multiple columns in one row
SELECT 1 AS a, 2 AS b, 'x' AS c;

-- Reading from a table (here, a system catalog)
SELECT relname, relkind
FROM pg_class
WHERE relname = 'pg_database'
LIMIT 1;
```

Each `SELECT` produces a result set. `AS column_name` renames the output column — without it, PostgreSQL invents a name like `?column?` for literals or uses the source column name. The arithmetic line is worth a second look: `10 / 4` returns the integer `2` because both operands are integers, while `10 / 4.0` returns `2.5` because one operand is numeric and integer division does not apply. This trap shows up often when computing averages and ratios.

`pg_class` is the system catalog that lists *relations* — tables, indexes, views, sequences, materialized views, and more. The `relkind` column distinguishes them (`r` = ordinary table, `i` = index, `v` = view, `m` = materialized view, `S` = sequence). Looking at catalogs is a habit worth developing; everything about your schema is visible there.

## Run the example {#run}

To run:

```text
$ psql -f source/select-basics.sql postgres
 one
-----
   1
(1 row)

 sum | quotient | real_quotient
-----+----------+---------------
   5 |        2 |            2.5
(1 row)

 greeting | chars
----------+-------
 hello    |     5
(1 row)

 a | b | c
---+---+---
 1 | 2 | x
(1 row)

   relname   | relkind
-------------+---------
 pg_database | r
(1 row)
```

## Common pitfalls {#pitfalls}

- Forgetting `AS` on an expression: `SELECT count(*)` works, but `SELECT count(*) FROM ... WHERE total > 0 GROUP BY id` gets unreadable without `AS order_count`. Always alias derived columns.
- Integer division: `1 / 2` returns `0`. Cast one side: `1::numeric / 2`, or `1.0 / 2`.
- Double-quotes vs. single-quotes: `'hello'` is a string literal; `"hello"` is an identifier (a column or table name). Mixing them up is one of the most common SQL beginner errors.

> [!TIP] Tip
> You can mix kinds of columns freely: `SELECT id, name, id * 2 AS doubled, current_timestamp AS fetched_at FROM fruits;` is perfectly fine. The result columns appear in the order written and use the aliases as names; expression evaluation order is not guaranteed.

> [!NOTE] Try it
> Run `SELECT 10 * 2 AS twenty;`. Then try `SELECT relname FROM pg_class WHERE relkind = 'r' LIMIT 5;` to see five table names in your database. Add `, relkind` after `relname` and observe.

## Downloads {#downloads}

SQL file: [select-basics.sql](/examples/source/select-basics.sql) · [All lesson files](/examples/examples.zip) · [Setup guide](/en/#prerequisites)