SQL Two

Altering Tables

In SQL, 'ALTER TABLE' statements can be used to:

Add/Remove Columns

Alter Column Data Types

Rename Columns

Change Table Name

Adding/Removing Tables

Adding tables syntax is the following:

ALTER TABLE racers
ADD COLUMN team TEXT;

Table being altered

How table is being altered

Column name and data type

Syntax for removing, or dropping, column is the following:

ALTER TABLE racers
DROP COLUMN team;

Altering Data Type

The syntax for altering data types is the following:

ALTER TABLE racers
ALTER team
SET DATA TYPE VARCHAR(100);

Table being altered

This can also be done in a shorter syntax:

ALTER TABLE racers
ALTER team
TYPE VARCHAR(100);

Column being altered

New data type

Renaming Columns/Table

The syntax for renaming a column is the following:

ALTER TABLE racers
RENAME COLUMN team
TO "group";

The syntax for renaming a table is the following:

ALTER TABLE racers
RENAME TO bike_racers;

Table Relationships

When working with relational databases, one tables data will often relate to another tables data. There are three main types of table relationships:

One to One

One to Many

Many to Many

One to One

One to One relationships mean one row in one table relates to one row in another table.

One to Many

One to Many means that one row in one table can relate to many rows in another table.

Many to Many

Many to Many means many rows from one table can relate to many rows from another table.

Foreign Keys

So how do we create these table relationships? Through constraints known as foreign keys. Foreign keys allow us to reference another tables column, typically a column with a primary key.

CREATE TABLE bikes (
  bikeId SERIAL PRIMARY KEY,
  racerId INTEGER REFERENCES racers(racerId),
  type TEXT,
  color TEXT
);

Table being referenced

Column being referenced

Join Statements

Once table relationships have been created, join statements can be used to collect data from tables that relate to each other.

SELECT *
FROM racers
JOIN bikes ON racers.racer_id = bikes.racer_id;

First table to join

How the tables are joining

Second table to join

Subqueries

Subqueries are another way to join tables. They are doing be nesting a select statement within another select statement:

SELECT *
FROM racers
WHERE racerid IN (
	SELECT racerid
  FROM bikes
  WHERE type = 'fezzari'
  AND age >= 25
)

SQL Two

By Matthew Bodily

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SQL Two

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