sql_cheat_sheet.md

Software Carpentry SQL Cheat Sheet

Basic Queries

Select one or more columns of data from a table:

SELECT column_name_1, column_name_2 FROM table_name;

Select all of the columns in a table:

SELECT * FROM table_name;

Get only unique lines in a query:

SELECT DISTINCT column_name FROM table_name;

Perform calculations in a query:

SELECT column_name_1, ROUND(column_name_2 / 1000.0) FROM table_name;

Filtering

Select only the data meeting certain criteria:

SELECT * FROM table_name WHERE column_name = 'Hello World';

Combine conditions:

SELECT * FROM table_name WHERE (column_name_1 >= 1000) AND (column_name_2 = 'A' OR column_name_2 = 'B');

Sorting

Sort results using ASC for ascending order or DESC for descending order:

SELECT * FROM table_name ORDER BY column_name_1 ASC, column_name_2 DESC;

Missing Data

Use NULL to represent missing data.

NULL is neither true nor false. Operations involving NULL produce NULL, e.g., 1+NULL, 2>NULL, and 3=NULL are all NULL.

Test whether a value is null:

SELECT * FROM table_name WHERE column_name IS NULL;

Test whether a value is not null:

SELECT * FROM table_name WHERE column_name IS NOT NULL;

Grouping and Aggregation

Combine data into groups and calculate combined values in groups:

SELECT column_name_1, SUM(column_name_2), COUNT(*) FROM table_name GROUP BY column_name_1;

Joins

Join data from two tables:

SELECT * FROM table_name_1 JOIN table_name_2 ON table_name_1.column_name = table_name_2.column_name;

Combining Commands

SQL commands must be combined in the following order: SELECT, FROM, JOIN, ON, WHERE, GROUP BY, ORDER BY.

Creating Tables

Create tables by specifying column names and types. Include primary and foreign key relationships and other constraints.

CREATE TABLE survey(
        taken   INTEGER NOT NULL,
        person  TEXT,
        quant   REAL NOT NULL,
        PRIMARY KEY(taken, quant),
        FOREIGN KEY(person) REFERENCES person(ident)
);

Transactions

Put multiple queries in a transaction to ensure they are ACID (atomic, consistent, isolated, and durable):

BEGIN TRANSACTION;
  DELETE FROM table_name_1 WHERE condition;
  INSERT INTO table_name_2 values(...);
END TRANSACTION;

Programming

Execute queries in a general-purpose programming language by:

• loading the appropriate library
• creating a connection
• creating a cursor
• repeatedly:
  • execute a query
  • fetch some or all results
• disposing of the cursor
• closing the connection

Python example:

import sqlite3
connection = sqlite3.connect("database_name")
cursor = connection.cursor()
cursor.execute("...query...")
for r in cursor.fetchall():
    ...process result r...
cursor.close()
connection.close()