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LautaroDiharceLautaroDiharce
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SQLite tutorial contribution (ronreiter#704)
* The union statement * The Case Statement * Altering Tables * Advanced Joins * Subqueries * Logic Fixes * Indexing * INSERT INTO SELECT * String Operations * Views
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Diff for: ‎tutorials/learnsqlonline.org/en/Altering Tables.md

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Tutorial
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--------
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Unlike SQL-standard and other database systems, SQLite supports a very limited functionality of the ALTER TABLE statement.
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By using an SQLite ALTER TABLE statement, you can perform three actions:
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Rename a table.
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Add a new column to a table.
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Rename a column
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To rename a table, you use the following ALTER TABLE RENAME TO statement:
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.exec
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CREATE TABLE Customer ( name TEXT,customerID INTERGER, city TEXT, email TEXT);
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INSERT INTO Customer ( name, customerID, city,email) VALUES
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("John", 1, "Huston", NULL), ("Eric", 2, "Paris", "[email protected]"), ("Jessica",3, "Kiev", "[email protected]"),
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("Mike", 4, "Paris", "[email protected]"), ("Jeff", 5, NULL, "[email protected]"),("Ben", 6, NULL, "[email protected]");
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ALTER TABLE Customer
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RENAME TO Customer_Alter;
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SELECT *
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FROM Customer_Alter;
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Something important to consider is if a table is referenced by views or statements in triggers, you must manually change the definition of views and triggers
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You can use the SQLite ALTER TABLE statement to add a new column to an existing table. In this scenario, SQLite appends the new column at the end of the existing column list
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There are some restrictions on the new column:
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The new column cannot have a UNIQUE or PRIMARY KEY constraint.
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If the new column has a NOT NULL constraint, you must specify a default value for the column other than a NULL value.
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The new column cannot have a default of CURRENT_TIMESTAMP, CURRENT_DATE, and CURRENT_TIME, or an expression.
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If the new column is a foreign key and the foreign key constraint check is enabled, the new column must accept a default value NULL.
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.exec
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CREATE TABLE Customer ( name TEXT,customerID INTERGER, city TEXT, email TEXT);
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ALTER TABLE Customer
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ADD COLUMN PhoneNum text
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INSERT INTO Customer ( name, customerID, city,email) VALUES
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("John", 1, "Huston", NULL,1111), ("Eric", 2, "Paris", "[email protected]",2222), ("Jessica",3, "Kiev", "[email protected]",3333),
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("Mike", 4, "Paris", "[email protected]",4444), ("Jeff", 5, NULL, "[email protected]",5555),("Ben", 6, NULL, "[email protected]",6666);
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SELECT *
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FROM Customer;
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SQLite added the support for renaming a column using ALTER TABLE RENAME COLUMN statement in version 3.20.0. Syntax is as follows:
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.exec
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CREATE TABLE Customer ( name TEXT,customerID INTERGER, city TEXT, email TEXT);
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INSERT INTO Customer ( name, customerID, city,email) VALUES
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("John", 1, "Huston", NULL), ("Eric", 2, "Paris", "[email protected]"), ("Jessica",3, "Kiev", "[email protected]"),
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("Mike", 4, "Paris", "[email protected]"), ("Jeff", 5, NULL, "[email protected]"),("Ben", 6, NULL, "[email protected]");
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ALTER TABLE Customer
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RENAME COLUMN city TO location;
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SELECT *
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FROM Customer;
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Exercise
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--------
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Rename the suppliers table and show its contents
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Tutorial Code
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-------------
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CREATE TABLE Supplier ( name TEXT,customerID INTERGER, city TEXT, stock INTERGER);
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INSERT INTO Supplier ( name, customerID, city, stock) VALUES
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("John", 1, "Huston"1000), ("Eric", 2, "Paris",200),
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("Mike", 3, "Paris", 20000),("Ben", 4, "Tokio",500);
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-- write your code here
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Expected Output
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---------------
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"John", 1, "Huston", 1000
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"Mike", 2, "Paris", 200
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"Eric", 3, "Kiev", 20000
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"Ben", 4, "Tokio", 500
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Solution
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--------
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CREATE TABLE Supplier ( name TEXT,customerID INTERGER, city TEXT, stock INTERGER);
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INSERT INTO Supplier ( name, customerID, city, stock) VALUES
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("John", 1, "Huston"1000), ("Eric", 2, "Paris",200),
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("Mike", 3, "Paris", 20000),("Ben", 4, "Tokio",500);
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-- write your code here
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ALTER TABLE Customer
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RENAME TO Customer_Alter;
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SELECT *
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FROM Customer_Alter;

Diff for: ‎tutorials/learnsqlonline.org/en/Combining INSERT INTO and SELECT.md

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Tutorial
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--------
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The INSERT INTO SELECT statement copies data from one table and inserts it into another table, the statement requires that the data types in source and target tables match.
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Note that the records in the target table are unaffected.
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The following is an example of this statement. Note that is specially useful if you want to do backups of tables.
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.exec
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CREATE TABLE contacts (
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first_name text NOT NULL,
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last_name text NOT NULL,
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email text NOT NULL
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);
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INSERT INTO contacts (first_name, last_name, email)
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VALUES('David','Brown','[email protected]'),
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('Johny','Doe','[email protected]'),
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('Lisa','Smith','[email protected]');
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CREATE TABLE contacts_bkp (
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first_name text NOT NULL,
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last_name text NOT NULL,
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email text NOT NULL
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);
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INSERT INTO contacts_bkp
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select * from contacts
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Additionally, you can add a WHERE clause to the query to filter the target dataset
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.exec
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CREATE TABLE contacts (
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first_name text NOT NULL,
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last_name text NOT NULL,
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email text NOT NULL,
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gender text NOT NULL
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);
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INSERT INTO contacts (first_name, last_name, email, gender)
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VALUES('David','Brown','[email protected]','M'),
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('Johny','Doe','[email protected]','M'),
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('Lisa','Smith','[email protected]','F');
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CREATE TABLE contacts_bkp (
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first_name text NOT NULL,
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last_name text NOT NULL,
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email text NOT NULL
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gender text NOT NULL
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);
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INSERT INTO contacts_bkp
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select * from contacts
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Where gender like 'F'
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Exercise
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--------
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Create a table and fill it just with the customer's table records, which names starts with the letter J
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Tutorial Code
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-------------
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CREATE TABLE Customer ( name TEXT,customerID INTERGER, city TEXT, email TEXT);
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INSERT INTO Customer ( name, customerID, city,email) VALUES
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("John", 1, "Huston", [email protected]), ("Eric", 2, "Paris", "[email protected]"), ("Jessica",3, "Kiev", "[email protected]"), ("Mike", 4, "Paris", "[email protected]"), ("Jeff", 5, Antananarivo, "[email protected]"),("Ben", 6, Antananarivo, "[email protected]");
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-- write your code here
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Expected Output
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---------------
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"John", 1, "Huston", [email protected]"
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"Jessica",3, "Kiev", "[email protected]"
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"Jeff", 5, Antananarivo, "[email protected]"
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Solution
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--------
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CREATE TABLE Customer ( name TEXT,customerID INTERGER, city TEXT, email TEXT);
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INSERT INTO Customer ( name, customerID, city,email) VALUES
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("John", 1, "Huston", [email protected]), ("Eric", 2, "Paris", "[email protected]"), ("Jessica",3, "Kiev", "[email protected]"), ("Mike", 4, "Paris", "[email protected]"), ("Jeff", 5, Antananarivo, "[email protected]"),("Ben", 6, Antananarivo, "[email protected]");
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-- write your code here
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CREATE TABLE Customer_J ( name TEXT,customerID INTERGER, city TEXT, email TEXT);
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INSERT INTO Customer_J
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Select * From Customer
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Where name like 'J%';
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Select * from Customer_J;

Diff for: ‎tutorials/learnsqlonline.org/en/Indexing.md

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Tutorial
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--------
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### What is an index?
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In relational databases, a table is a list of rows. In the same time, each row has the same column structure that consists of cells. Each row also has a consecutive row-id sequence number used to identify the row. Therefore, you can consider a table as a list of pairs: (rowid, row).
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Unlike a table, an index has an opposite relationship: (row, rowid). An index is an additional data structure that helps improve the performance of a query.
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SQLite uses B-tree for organizing indexes. Note that B stands for balanced, B-tree is a balanced tree, not a binary tree.
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The B-tree keeps the amount of data at both sides of the tree balanced so that the number of levels that must be traversed to locate a row is always in the same approximate number. In addition, querying using equality (=) and ranges (>, >=, <,<=) on the B-tree indexes are very efficient.
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### SQLite CREATE INDEX statement
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To create an index, you use the CREATE INDEX statement with the following syntax:
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CREATE [UNIQUE] INDEX index_name
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ON table_name(column_list);
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To create an index, you specify three important information:
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- The name of the index after the CREATE INDEX keywords.
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- The name of the table to the index belongs.
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- A list of columns of the index.
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In case you want to make sure that values in one or more columns are unique like email and phone, you use the UNIQUE option in the CREATE INDEX statement. The CREATE UNIQUE INDEX creates a new unique index.
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Suppose, you want to enforce that the email is unique, you create a unique index as follows:
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.exec
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CREATE TABLE contacts (
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first_name text NOT NULL,
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last_name text NOT NULL,
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email text NOT NULL
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);
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CREATE UNIQUE INDEX idx_contacts_email
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ON contacts (email);
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INSERT INTO contacts (first_name, last_name, email)
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VALUES('David','Brown','[email protected]'),
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('Johny','Doe','[email protected]'),
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('Lisa','Smith','[email protected]');
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If you want to insert a duplicated row, an error message will appear, indicating that the unique index has been violated. Because when you inserted the second row, SQLite checked and made sure that the email is unique across of rows in email of the contacts table.
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If you query data from the contacts table based on a specific email, SQLite will use the index to locate the data
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.exec
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CREATE TABLE contacts (
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first_name text NOT NULL,
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last_name text NOT NULL,
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email text NOT NULL
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);
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CREATE UNIQUE INDEX idx_contacts_email
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ON contacts (email);
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INSERT INTO contacts (first_name, last_name, email)
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VALUES('David','Brown','[email protected]'),
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('Johny','Doe','[email protected]'),
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('Lisa','Smith','[email protected]');
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SELECT
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first_name,
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last_name,
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email
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FROM
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contacts
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WHERE
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email = '[email protected]';
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To check if SQLite uses the index or not, you use the EXPLAIN QUERY PLAN
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.exec
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CREATE TABLE contacts (
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first_name text NOT NULL,
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last_name text NOT NULL,
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email text NOT NULL
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);
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CREATE UNIQUE INDEX idx_contacts_email
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ON contacts (email);
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INSERT INTO contacts (first_name, last_name, email)
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VALUES('David','Brown','[email protected]'),
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('Johny','Doe','[email protected]'),
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('Lisa','Smith','[email protected]');
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EXPLAIN QUERY PLAN
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SELECT
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first_name,
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last_name,
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email
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FROM
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contacts
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WHERE
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email = '[email protected]';
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### SQLite multicolumn index
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If you create an index that consists of one column, SQLite uses that column as the sort key. In case you create an index that has multiple columns, SQLite uses the additional columns as the second, third, … as the sort keys.
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SQLite sorts the data on the multicolumn index by the first column specified in the CREATE INDEX statement. Then, it sorts the duplicate values by the second column, and so on.
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Therefore, the column order is very important when you create a multicolumn index.
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To utilize a multicolumn index, the query must contain the condition that has the same column order as defined in the index.
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### SQLite DROP INDEX statement
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To remove an index from a database, you use the DROP INDEX statement as follows:
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DROP INDEX [IF EXISTS] index_name;
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In this syntax, you specify the name of the index that you want to drop after the DROP INDEX keywords. The IF EXISTS option removes an index only if it exists.
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Exercise
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--------
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Create an index for the following table, index by customerID. Then select all.
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Tutorial Code
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-------------
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CREATE TABLE Customer ( name TEXT,customerID INTERGER, city TEXT, email TEXT);
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INSERT INTO Customer ( name, customerID, city,email) VALUES
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("John", 1, "Huston", [email protected]), ("Eric", 2, "Paris", "[email protected]"), ("Jessica",3, "Kiev", "[email protected]"), ("Mike", 4, "Paris", "[email protected]"), ("Jeff", 5, Antananarivo, "[email protected]"),("Ben", 6, Antananarivo, "[email protected]");
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-- write your code here
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Expected Output
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---------------
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"John", 1, "Huston", [email protected]"
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"Eric", 2, "Paris", "[email protected]"
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"Jessica",3, "Kiev", "[email protected]"
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"Mike", 4, "Paris", "[email protected]"
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"Jeff", 5, Antananarivo, "[email protected]"
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"Ben", 6, Antananarivo, "[email protected]"
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Solution
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--------
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CREATE TABLE Customer ( name TEXT,customerID INTERGER, city TEXT, email TEXT);
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INSERT INTO Customer ( name, customerID, city,email) VALUES
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("John", 1, "Huston", [email protected]), ("Eric", 2, "Paris", "[email protected]"), ("Jessica",3, "Kiev", "[email protected]"), ("Mike", 4, "Paris", "[email protected]"), ("Jeff", 5, Antananarivo, "[email protected]"),("Ben", 6, Antananarivo, "[email protected]");
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-- write your code here
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CREATE UNIQUE INDEX idx_Cus_id
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ON Customer (customerID);
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Select * from Customer

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