SQL: Unique and primary keys

With a table created with this command:

CREATE TABLE people (
  age INTEGER NOT NULL,
  name TEXT NOT NULL
);

We can have columns that repeat the same value. The DBMS does not care.

But, we can force a column to have only unique values using the UNIQUE key constraint:

DROP TABLE people;

CREATE TABLE people (
  age INTEGER NOT NULL,
  name TEXT NOT NULL UNIQUE
);

Now if you try to add the ‘Flavio’ twice:

INSERT INTO people (age, name) VALUES (39, 'Flavio');
INSERT INTO people (age, name) VALUES (38, 'Flavio');

You’d get an error:

A primary key is a unique key that has another property: it’s the primary way we identify a row in the table.

What we usually do (but not always) is we use the SERIAL type for the primary key, which means it’s a unique INTEGER value that we assign to each record and we increment it automatically any time we add as new record.

DROP TABLE people;

CREATE TABLE people (
  id SERIAL PRIMARY KEY,
  age INTEGER NOT NULL,
  name TEXT NOT NULL
);

So when we add an item we can omit its value, because the DBMS knows its value:

INSERT INTO people (age, name) VALUES (39, 'Flavio');
INSERT INTO people (age, name) VALUES (20, 'Roger');
id | age |  name
---+-----+------
1  | 39 |  'Flavio'
2  | 20 |  'Roger'

It’s important to define a primary key so:

  • we always know we can uniquely identify a row in the table
  • we can create relationships between tables, so a table can reference a row into another table
  • we can have data integrity and referential integrity (something we’ll not talk about, but it’s important to note)
  • and last but not least, it’s important for speed, so the database can figure out how to optimize your table

Lessons in this unit:

0: Introduction
1: Using TablePlus
2: Create a Postgres database
3: Create a new table
4: Add data to a table
5: Query data with SELECT
6: Delete data from the database
7: Update data with UPDATE
8: Null constraints
9: ▶︎ Unique and primary keys
10: Updating a table structure
11: Joins
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