Chapter 4. Using SQL with PostgreSQL

Table of Contents
Introduction to psql
Using Tables
Adding Data with INSERT and COPY
Retrieving Rows with SELECT
Modifying Rows with UPDATE
Removing Rows with DELETE
Using Sub-Queries
Using Views
Further SQL Application

This chapter continues the discussion of SQL begun in the last chapter, this time with a practical focus. It addresses the creation of tables, the population of those tables with data, and the managing of that data via SQL statements.

Like most network-capable database systems, PostgreSQL fits into a client-server paradigm. The heart of PostgreSQL is the server backend, or the postmaster process. It is called a "backend" because it is not meant to be directly interfaced with by a user; rather, it can be connected to with one of a variety of clients.

When you start the PostgreSQL service, the postmaster process starts running in the background, listening to a specific TCP/IP port for connections from clients. Unless explicitly configured, postmaster will bind to, and listen on, port 5432.

There are several interfaces available for client connectivity to the postmaster process. In the provided examples, this book will make use of psql, which is the most portable and readily accessed client distributed with PostgreSQL.

This chapter covers the basics of how to use psql, how to create and use tables, and how to retrieve and manage data within those tables. It also addresses the topics of SQL sub-queries and views.

Introduction to psql

The psql client is a command-line client distributed with PostgreSQL, and is often called the interactive monitor or interactive terminal. With psql, you are provided a simple yet powerful tool with which you can directly interface with the PostgreSQL server, and thereby begin exploring SQL.

Starting psql

Before starting psql, be sure that you have either copied the psql binary into a path which is in your system PATH variable (e.g., /usr/bin), or that you have placed the PostgreSQL binary path (e.g., /usr/local/pgsql/bin) within your list of paths in your PATH environment variable (as shown in Chapter 2).

How you set the appropriate PATH variable will depend on your system shell. An example in either bash or ksh might read:

$ export PATH=$PATH:/usr/local/pgsql/bin

An example in either csh or tcsh might read:

$ set path=($path /usr/local/pgsql/bin)

Example 4-1. Setting System Path for psql

[user@host user]$ psql
bash: psql: command not found
[user@host user]$ echo $PATH
[user@host user]$ export PATH=$PATH:/usr/local/pgsql/bin
[user@host user]$ psql testdb
Welcome to psql, the PostgreSQL interactive terminal.

Type:  \copyright for distribution terms
       \h for help with SQL commands
       \? for help on internal slash commands
       \g or terminate with semicolon to execute query
       \q to quit


Note that Example 4-1 is demonstrated within a Bash shell.

Once you have appropriately set your PATH variable, you should be able to type psql, along with a database name, to start up the PostgreSQL interactive terminal.


Remember that shell environment variables are erased after you have logged out. If you wish for your changes to the PATH variable to be persistent on logging in, you will need to enter the appropriate PATH declaration into your shell-specific start-up scripts (e.g., ~/.bash_profile).

Introduction to psql Syntax

When you first start psql, you will be greeted with a brief synopsis of four essential psql slash commands: \h for SQL help, \? for help on psql-specific commands, \g for executing queries and \q for actually exiting psql once you are done.

Every psql-specific command is prefixed by a backslash; hence the term "slash command" used above. For a complete list of slash commands and a brief description their functions, type \? into the psql command line, and press enter.

Example 4-2. Listing psql Slash Commands

booktown=# \?
 \a             toggle between unaligned and aligned mode
 \c[onnect] [dbname|- [user]]
                connect to new database (currently 'booktown')
 \C <title>     table title
 \copy ...      perform SQL COPY with data stream to the client machine
 \copyright     show PostgreSQL usage and distribution terms
 \d <table>     describe table (or view, index, sequence)
 \d{t|i|s|v}    list tables/indices/sequences/views
 \d{p|S|l}      list permissions/system tables/lobjects
 \da            list aggregates
 \dd [object]   list comment for table, type, function, or operator
 \df            list functions
  \do            list operators
 \dT            list data types          be duplicated
 \e [file]      edit the current query buffer or [file] with external editor
 \echo <text>   write text to stdout
 \encoding <encoding>  set client encoding
 \f <sep>       change field separator
 \g [file]      send query to backend (and results in [file] or |pipe)
 \h [cmd]       help on syntax of sql commands, * for all commands
 \H             toggle HTML mode (currently off)
 \i <file>      read and execute queries from <file>
 \l             list all databases
 \lo_export, \lo_import, \lo_list, \lo_unlink
                large object operations
 \o [file]      send all query results to [file], or |pipe
 \p             show the content of the current query buffer
 \pset <opt>    set table output  <opt> = {format|border|expanded|fieldsep|
 \q             quit psql
 \qecho <text>  write text to query output stream (see \o)
 \r             reset (clear) the query buffer
 \s [file]      print history or save it in [file]
 \set <var> <value>  set internal variable
 \t             show only rows (currently off)
 \T <tags>      HTML table tags
 \unset <var>   unset (delete) internal variable
 \w <file>      write current query buffer to a <file>
 \x             toggle expanded output (currently off)
 \z             list table access permissions
 \! [cmd]       shell escape or command

Executing Queries

Entering and executing queries within psql can be done two different ways. When using the client in interactive mode, the normal method is to directly enter queries into the prompt (i.e., standard input, or stdin). However, through the use of psql's \i slash command, you can have psql read and interpret a file on your local filesystem as the query data.

Entering queries at the psql prompt

To enter queries directly into the prompt, open psql and make sure you are connected to the correct database (and logged in as the correct user). You will be presented with a prompt that, by default, is set to display the name of the database you are currently connected to. The prompt will look like this: psql:


To pass SQL statements to PostgreSQL, simply type them into the prompt. Anything you type (barring a slash command) will be queued until you terminate the query with a semi-colon. This is the case even if you start a new line of type, thus allowing you to spread query statements across multiple lines. Examine Example 4-3 to see how this is done.

Example 4-3. Entering Statements into psql

testdb=# SELECT * FROM employees
testdb-#          WHERE firstname = 'Michael';

The query entered in Example 4-3 will return a table that consists of all employees whose first name is Michael. The query could be broken up over multiple lines to improve readability, and psql would not send it to the backend until the terminating semi-colon was sent. The prompt will show the end-character of a previous line if the character requires a closing character, such as a parenthesis or a quote (this is not shown in the example). If you were to issue a CREATE TABLE command to start a statement, and then hit enter to begin a new line for readability purposes, you would see a prompt similar to the one displayed in Example 4-4.

Example 4-4. Leaving end-characters open

testdb=# CREATE TABLE employees (

At this point you could continue the statement. The psql prompt is informing you of the open parenthesis by inserting an open parenthesis symbol into the prompt.

Editing the query buffer

Use the \e command to edit the current query buffer with the editor that your EDITOR environment variable is set to. Doing so can be very useful when entering queries and statements in psql, as you can easily view and modify all lines of your query or statement before it is committed. Example 4-5 shows how to set the EDITOR variable. The vi editor will be used if EDITOR is not set.

Example 4-5. Setting the EDITOR Variable

$ set EDITOR='joe'
$ export EDITOR

You can also use this command to save your current buffer as a file. Issue the \e command to enter editing mode. This will open your editor and load the buffer as if it were a file. Complete whatever work you wish to do with the buffer, then use your editor's save function to save the buffer and return to psql. To save the query as a normal file, use your editor's save-as function and save it as a file other than the .tmp created by \e.