+
SQL Cheat Sheet
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Last updated on 2023-10-19 |
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Basic Queries
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Select one or more columns of data from a table:
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+
SH
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SELECT column_name_1, column_name_2 FROM table_name;
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Select all of the columns in a table:
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SH
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SELECT * FROM table_name;
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Get only unique lines in a query:
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SH
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SELECT DISTINCT column_name FROM table_name;
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Perform calculations in a query:
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SH
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SELECT column_name_1, ROUND(column_name_2 / 1000.0) FROM table_name;
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Filtering
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Select only the data meeting certain criteria:
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SH
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SELECT * FROM table_name WHERE column_name = 'Hello World';
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Combine conditions:
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SH
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SELECT * FROM table_name WHERE (column_name_1 >= 1000) AND (column_name_2 = 'A' OR column_name_2 = 'B');
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Sorting
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Sort results using ASC for ascending order or
+DESC for descending order:
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SH
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SELECT * FROM table_name ORDER BY column_name_1 ASC, column_name_2 DESC;
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Missing Data
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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:
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SH
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SELECT * FROM table_name WHERE column_name IS NULL;
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Test whether a value is not null:
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SH
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SELECT * FROM table_name WHERE column_name IS NOT NULL;
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Grouping and Aggregation
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Combine data into groups and calculate combined values in groups:
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SH
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SELECT column_name_1, SUM(column_name_2), COUNT(*) FROM table_name GROUP BY column_name_1;
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Joins
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Join data from two tables:
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SH
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SELECT * FROM table_name_1 JOIN table_name_2 ON table_name_1.column_name = table_name_2.column_name;
+
Combining Commands
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SQL commands must be combined in the following order:
+SELECT, FROM, JOIN,
+ON, WHERE, GROUP BY,
+ORDER BY.
+Creating Tables
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Create tables by specifying column names and types. Include primary
+and foreign key relationships and other constraints.
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+
SH
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CREATE TABLE survey(
+ taken INTEGER NOT NULL,
+ person TEXT,
+ quant REAL NOT NULL,
+ PRIMARY KEY(taken, quant),
+ FOREIGN KEY(person) REFERENCES person(ident)
+);
+
Transactions
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Put multiple queries in a transaction to ensure they are ACID
+(atomic, consistent, isolated, and durable):
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+
SH
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BEGIN TRANSACTION;
+ DELETE FROM table_name_1 WHERE condition;
+ INSERT INTO table_name_2 values(...);
+END TRANSACTION;
+
Programming
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Execute queries in a general-purpose programming language by:
+- loading the appropriate library
+- creating a connection
+- creating a cursor
+- repeatedly:
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- execute a query
+- fetch some or all results
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+- disposing of the cursor
+- closing the connection
+
Python example:
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+
SH
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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()
+