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
)Actual SQL Two
By awestenskow
