The PostgreSQL Cheat Sheet (Part 2)

Written by Data Pilot
December 02, 2019
  • PostgreSQL

Introduction to the PostgreSQL cheat sheet

The PostgreSQL offers scaling, storage flexibility, and ease of database management for developers, DBAs, and other technical professionals. Because of PostgreSQL’s ability to offer architecture stability along with an extensive coding capability, the plethora of commands and statements are endless. That’s a good thing. What’s even better is having a handy list of the SQL commands you’re likely to use regularly. Well, some of the most popular ones are featured here in Part 2 of The PostgreSQL Cheat Sheet, so take a few moments to review it now.

Prerequisites to using PostgreSQL SQL commands in psql

  • Be sure that the object-relational database management systemPostgreSQL installed on your OS.

  • At the command line psql, check the PostgreSQL version with the command psql -V.

  • You’ll also need PostgreSQL database accessibility to try out the samples shown in this PostgreSQL cheat sheet.

Screenshot of a terminal window getting the PostgreSQL server status and psql version

NOTE: Here are some useful tips regarding commands. When writing code in psql, always end your SQL statement with a semicolon (;). If you don’t and push Return, your code will extend to the next line of code without breaking at the place where you wanted it to end.

Another tip for writing SQL statements is to remember to enclose your strings in PostgreSQL with singular quotes, not doubles. This one is correct: 'string here' for example. A syntax error will happen if you use doubles.

Finally, to quickly get away from a long results list or a command you started or completed, push CTRL+C.

Accessing PostgreSQL using the ‘psql’ command line interface

  • From your server on your localhost, connect to your database in PostgreSQL with the command psql.
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psql postgres
  • Alternatively to the above command, input your username, host, and then database name to make a Postgres database connection.
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psql -U some_username -h 127.0.0.1 -d some_database

NOTE: See the flags in the above code.

  • Your username in Postgres comes after the flag -U
  • The IP address or host domain goes after the flag -h
  • The database PostgreSQL name is inputted after the flag -d

PostgreSQL cheat sheet of useful SQL queries and commands

This PostgreSQL cheat sheet contains some of the most frequently-used commands to perform basic computing software programming functions so that you can code with efficiency.

Use ‘SELECT’ to get a Postgres table’s column names

  • Obtain the names of a table’s columns with the information_schema command:
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SELECT * FROM information_schema.columns
WHERE some_table = 'some_table';
  • You can also access the names of a public table’s column with the table_schema command:
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SELECT * FROM information_schema.columns
WHERE table_schema = 'public'
AND some_table = 'some_table';

PostgreSQL cheat sheet commands for modifying tables

  • Use the INSERT INTO statement to add a value to a table:
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INSERT INTO some_table(col1, col2) VALUES(value1,value2);

NOTE: The above command adds two columns and two values. See (col1, col2) and (value1, value2) in the SQL statement. Place a comma after each indicated column or value within the parenthesis when you have more than one to add to a table.

  • Use the INSERT INTO and SELECT statement to add a table’s column data to a different table:
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INSERT INTO table1(column_list) SELECT column_list FROM table2;
  • Save table updates with the UPDATE statement:
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UPDATE some_table SET col1 = value1;
  • Use the UPDATE statement for condition matching in a table:
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UPDATE some_table SET col1 = new_val, col2 = new_val WHERE condition;
  • Use the DELETE FROM statement to remove all records from a table in Postgres:
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DELETE FROM some_table;

NOTE: An option to remove all records from a table in Postgres is to use the command TRUNCATE followed by naming the table you want to delete.

  • Use the DELETE FROM statement to remove data pertaining to a condition:
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DELETE FROM some_table
WHERE condition;

PostgreSQL cheat sheet for managing databases

  • Make a database with the CREATE DATABASE statement:
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CREATE DATABASE [IF NOT EXISTS] db_name;

NOTE: A database may already exist, so to avoid raising an exception, use the IF NOT EXISTS clause after the CREATE DATABASE statement.

  • Use DROP DATABASE to delete a database forever:
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DROP DATABASE [IF NOT EXISTS] db_name;

NOTE: You won’t get an error message prompt if you add the IF NOT EXISTS clause.

Use the a PostgreSQL ‘SELECT’ statement to query data

Query data in a table with these various SELECT statement command.

  • Add the wildcard asterisk * symbol to have every record in a PostgreSQL table return in the results page.
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SELECT * FROM some_table;
  • Indicate which columns to query:
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SELECT col1, col2 FROM some_table;
  • Query a filtered table:
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SELECT * FROM some_table WHERE condition;
  • Include the clause WHERE to specify the columns you want to query:
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SELECT some_col, another_col FROM some_table
WHERE some_int > 50;

Screenshot of psql using SQL SELECT keyword to return records with WHERE clause

  • Perform a column query and give the column an alias with this statement:
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SELECT col1 AS new_column FROM some_table;

Here are some commands to query in PostgreSQL using operators.

  • Use the operator LIKE to query a character string pattern match:

To query a set of operations in PostgreSQL:

Using the LIKE operator:

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SELECT * FROM some_table
WHERE COLUMN LIKE '%value';
  • Use the operator BETWEEN to query a table range:
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SELECT * FROM some_table WHERE COLUMN BETWEEN low AND high;
  • Use the operator IN to add more than one condition or value to your WHERE clause in your statement:
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SELECT * FROM some_table WHERE COLUMN IN (value1, value2);
  • Use the operator UNION to merge at least two SELECT statement results sets.
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SELECT FROM table1 UNION SELECT * FROM table2;
  • Use the operator EXCEPT to put together two SELECT statements that will only return rows that are not in the second statement.
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SELECT * FROM table1 EXCEPT SELECT * FROM table2;
  • Use the operator INTERSECT to have the results set to reflect every record picked by at least two statements. If a record fails to match each query, it won’t appear in the results set:
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SELECT * FROM table1 INTERSECT SELECT * FROM table2;
  • Use the clause LIMIT to return a limited amount of rows. In the statement below, OFFSET rows are skipped:
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SELECT * FROM some_table
LIMIT LIMIT OFFSET offset
ORDER BY column_name;

Here are a few SQL statements for querying multiple tables.

  • Make a multiple Postgres table query with the INNER JOIN statement:
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SELECT * FROM table1 INNER JOIN table2 ON conditions
  • Make a multiple Postgres table query with the LEFT JOIN statement:
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SELECT * FROM table1 LEFT JOIN table2 ON conditions
  • Make a multiple Postgres table query with the FULL OUTER JOIN statement:
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SELECT * FROM table1 FULL OUTER JOIN table2 ON conditions
  • Make a multiple Postgres table query with the CROSSJOIN statement:
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SELECT * FROM table1 CROSS JOIN table2 ON conditions
  • Make a multiple Postgres table query with the NATURAL JOIN statement:
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SELECT * FROM table1 NATURAL JOIN table2 ON conditions

Here are a few common SELECT statements for displaying table rows.

  • Use the wildcard (*) to show all table rows:
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SELECT COUNT(*) FROM some_table;
  • Use the ORDER BY clause to sort the order of table rows in the results:
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SELECT column_name FROM some_table ORDER BY column_name [ASC|DESC];
  • To group table data, use the clause GROUP BY:
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SELECT * FROM some_table GROUP BY col1, col2;
  • Use both clauses HAVING and GROUP BY to specify the criteria for grouping the data results:
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SELECT * FROM some_table GROUP BY col1 HAVING condition;

Conclusion on the PostgreSQL cheat sheet

A helpful PostgreSQL cheat sheet is meant to help you reduce the time you spend on your daily coding projects. Make it an accessible reference of common SQL statements and other commands are at your fingertips. This way, you won’t have to wonder if the syntax is off the mark. The most beneficial result is that you’ll likely cut down on unnecessarily raised exceptions every day.

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