How to perform basic queries with `SELECT` in PostgreSQL

Introduction

The SELECT command is the primary way to query and read information about records stored within database tables within PostgreSQL. Its usefulness, however, is not restricted to read-only operations. The SELECT syntax is combined with many other commands to target specific records or fields within databases for updates, deletions, and more complex operations.

In this guide, we'll show how the basic syntax of SELECT supports gathering data from tables. While we'll leave the vast number of optional clauses to the command for other articles, it will hopefully become evident how even the most basic components provide a strong foundation for querying data. These fundamentals only require you to learn a few clauses and constructions.

The general syntax of the SELECT command

The basic format of a SELECT command looks something like this:

SELECT <column_names> FROM <table_name> <additional_conditions_and_formatting>;

This statement is composed of a few different pieces:

• SELECT: The SELECT command itself. This SQL statement indicates that we want to query tables or views for data they contains. The arguments and clauses surrounding it determine both the contents and the format of the output by defining criteria.
• <column_names>: The SELECT statement can return entire rows (indicated by the * wildcard character) or a subset of the available columns. If you want to output only specific columns, provide the column names you'd like to display, separated by commas.
• FROM <table_name>: The FROM keyword is used to indicate the table or view that should be queried. In most simple queries, this consists of a single table that contains the data you're interested in.
• <additional_conditions_and_formatting>: A large number of filters, output modifiers, and conditions can be specified as additions to the SELECT command. You can use these to help pinpoint data with specific properties, modify the output formatting, or further process the results.

Specifying columns to display with SELECT

The column specification portion of the SELECT command requires you to name the columns you want to display for the data you are querying.

For ad hoc querying and during data exploration, one of the most helpful options is to use an asterisk to indicate that you want to display values from every column available:

SELECT * FROM my_table;

This will display all of the records from my_table since we do not provide any filtering to narrow the results. All of the columns for each record will be shown in the order that they are defined within the table.

You can also choose to view a subset of available column by specifying them by name. Column names are separated by commas and are displayed in the order in which they are given:

SELECT column2, column1 FROM my_table;

This will display all of the records from my_table, but only show the columns named column2 and column1, in that order.

Using column aliases with AS to modify the resulting table

You can optionally set column aliases to modify the name used for columns in the output.

SELECT column1 AS "first column" FROM my_table;

This will show the each of the values for column1 in my_table. However, the column in the output will be labeled as first column instead of column1.

This is especially useful if the output combines column names from multiple tables that might share names or if it includes computed columns that don't already have a name.

Defining sort order with ORDER BY

The ORDER BY clause can be used to sort the resulting rows according to the criteria given. The general syntax looks like this:

SELECT * FROM my_table ORDER BY <sort_expression>;

This will display the values for all columns in all records within my_table. The results will be ordered according to the expression represented by the placeholder <sort_expression>.

For example, suppose we have a customer table that contains columns for first_name, last_name, address, and phone_number. If we want to display the results in alphabetical order by last_name, we could use the following command:

SELECT * FROM customer ORDER BY last_name;

The results are sorted in ascending alphabetical order by the last_name column.

To reverse the ordering, we can add the DESC modifier to the end of the ORDER BY clause:

SELECT * FROM customer ORDER BY last_name DESC;

You can also sort by multiple columns. Here, we sort first by last_name, and then by first_name for any columns with the same last_name value. Both sorts are in ascending order:

SELECT * FROM customer ORDER BY last_name, first_name;

One additional option that is often important is clarifying where NULL values should be presented in the sort order. You can do this by adding NULLS FIRST (the default) or NULLS LAST for any sort column:

SELECT * FROM customer ORDER BY last_name NULLS LAST;

Getting distinct results

If you want to find the range of values for a column in PostgreSQL, you can use the SELECT DISTINCT variant. This will display a single row for each distinct value of a column.

The basic syntax looks like this:

SELECT DISTINCT column1 FROM my_table;

This will show one row per unique value in column1.

For example, to display all of the different values for color that your shirt table contains, you can type:

SELECT DISTINCT color FROM shirt;

color
------
blue
green
orange
red
yellow

To show uniqueness across multiple columns, you can add additional columns separated by commas.

For instance, this will display all of the different combinations of color and shirt_size for the shirt table:

SELECT DISTINCT color,shirt_size FROM shirt;

color  | shirt_size
-------+-----------
blue   | M
blue   | S
green  | M
green  | L
green  | S
orange | L
orange | M
red    | M
yellow | S

This displays every unique combination of color and shirt_size within the table.

An often more flexible variant is PostgreSQL's SELECT DISTINCT ON command. This format allows you to specify a list of columns that should be unique in combination and separately list the columns you wish to display.

The general syntax looks like this, with the column or columns that should be unique listed in the parentheses after SELECT DISTINCT ON, followed by the columns you wish to display:

SELECT DISTINCT ON (column1) column1, column2 FROM my_table ORDER BY column1;

For example, if you want to display a single color for each shirt size, you could type:

SELECT DISTINCT ON (shirt_size) color,shirt_size FROM shirt;

color | shirt_size
------+-----------
red   | M
green | L
green | S

This will show a single row for each unique value in shirt_size. For each row, it will display the color column, followed by the shirt_size column.

If using an ORDER BY clause, the column selected for ordering must match the column selected within the DISTINCT ON parentheses for the output to have predictable results:

SELECT DISTINCT ON (shirt_size) color,shirt_size FROM shirt ORDER BY shirt_size DESC;

color | shirt_size
------+-----------
green | S
red   | M
green | L

Conclusion

In this guide, we covered some of the basic ways you can use the SELECT command to identify and display records from your tables and views. The SELECT command is one of the most flexible and powerful operations within SQL-oriented databases, with many different ways to add clauses, conditions, and filtering.

While we only covered basic usage in this guide, the general format you learned here will serve as the foundation for all other read and many write queries. Learning ways to filter and target the results more accurately extend the capabilities that we covered today.