relational database - candidate key -
Published: 2024-04-09 08:13:55

```sql
CREATE TABLE League (
    TeamPosition INT,
    TeamName VARCHAR(100),
    State VARCHAR(100)
);

```

```sql
CREATE TABLE Teams (
    TeamName VARCHAR(100),
    Captain VARCHAR(100),
    Coach VARCHAR(100)
);
```

```sql
CREATE TABLE Points (
    TeamPosition INT,
    TeamName VARCHAR(100),
    Points INT
);
```

Based on the current Premier League standings and team details, here's how you could populate your tables with 4 rows of data:

```sql
-- Inserting data into the League table
INSERT INTO League (TeamPosition, TeamName, State)
VALUES
(1, 'Arsenal', 'London'),
(2, 'Liverpool', 'Liverpool'),
(3, 'Manchester City', 'Manchester'),
(4, 'Tottenham Hotspur', 'London');

-- Inserting data into the Teams table
INSERT INTO Teams (TeamName, Captain, Coach)
VALUES
('Arsenal', 'Martin Odegaard', 'Mikel Arteta'),
('Liverpool', 'Jordan Henderson', 'Jurgen Klopp'),
('Manchester City', 'Kevin De Bruyne', 'Pep Guardiola'),
('Tottenham Hotspur', 'Harry Kane', 'Antonio Conte');

-- Inserting data into the Points table
INSERT INTO Points (TeamPosition, TeamName, Points)
VALUES
(1, 'Arsenal', 71),
(2, 'Liverpool', 71),
(3, 'Manchester City', 70),
(4, 'Tottenham Hotspur', 60);
```

Please note that the `State` column in the `League` table typically refers to a U.S. state, but in the context of the Premier League, it's more appropriate to use the city or locality of the team. The points and positions are based on the latest standings³, and the team captains and coaches are based on the most recent information available⁹[^10^]. Remember to update these details as they can change over time due to transfers, managerial changes, and other factors.


In the context of relational databases, a **key attribute** is a column or a set of columns that uniquely identifies each row in a table. There are different types of key attributes:

- **Primary Key**: A unique identifier for each row in a table. No two rows can have the same primary key value.
- **Foreign Key**: A column that creates a relationship between two tables. The foreign key in one table refers to the primary key in another table.
- **Composite Key**: A combination of two or more columns that uniquely identifies each row in a table.

Based on the SQL statements you've provided, here's how the key attributes could be defined:

1. **League Table**: The `TeamPosition` could be the primary key as it uniquely identifies each team's position in the league. However, if teams can share the same position (in case of a tie), then `TeamPosition` should not be the primary key. Instead, `TeamName` could be used as the primary key if each team name is unique.

```sql
ALTER TABLE League
ADD PRIMARY KEY (TeamName);
```

2. **Teams Table**: The `TeamName` is likely the primary key here, assuming each team has a unique name.

```sql
ALTER TABLE Teams
ADD PRIMARY KEY (TeamName);
```

3. **Points Table**: Similar to the League table, `TeamPosition` could be the primary key. However, if the positions can change or be shared among teams, it might not be suitable as a primary key. Alternatively, `TeamName` could be the primary key if it's unique.

```sql
ALTER TABLE Points
ADD PRIMARY KEY (TeamName);
```

In a real-world scenario, you would typically have a unique identifier that doesn't change over time, like a `team_id`, to serve as the primary key. This would help maintain data integrity, especially when team names or positions change from season to season.

The `ALTER TABLE` statement in SQL is used to modify the structure of an existing table. It can be used for various purposes, such as:

- Adding, dropping, or modifying columns in the table.
- Adding or dropping constraints like primary keys, foreign keys, unique constraints, etc.
- Changing the data type of a column.

Here are some examples of how `ALTER TABLE` can be used:

**Adding a Column:**
```sql
ALTER TABLE table_name
ADD column_name data_type;
```

**Dropping a Column:**
```sql
ALTER TABLE table_name
DROP COLUMN column_name;
```

**Modifying a Column's Data Type:**
```sql
ALTER TABLE table_name
ALTER COLUMN column_name SET DATA TYPE new_data_type;
```

**Adding a Primary Key:**
```sql
ALTER TABLE table_name
ADD PRIMARY KEY (column_name);
```

**Adding a Foreign Key:**
```sql
ALTER TABLE table_name
ADD FOREIGN KEY (column_name) REFERENCES other_table (other_table_column);
```

Remember to replace `table_name`, `column_name`, `data_type`, `new_data_type`, `other_table`, and `other_table_column` with the actual table names, column names, and data types relevant to your database schema. Use `ALTER TABLE` with caution, as changes to the table structure can impact data integrity and application functionality. Always ensure you have a backup before making structural changes to a database.

**candidate key** 
In the context of relational databases, a **candidate key** is a set of attributes that uniquely identify rows in a table. A table may have multiple candidate keys, but each candidate key must satisfy two conditions:

1. **Uniqueness**: No two rows can have the same value of the candidate key.
2. **Minimality**: No subset of the candidate key can satisfy the uniqueness condition.

From the given tables, let's identify potential candidate keys:

1. **League Table**: The `TeamPosition` could be a candidate key if each team has a unique position. However, if positions can be shared (e.g., in case of a tie), then `TeamPosition` alone would not be a candidate key. `TeamName` could be another candidate key if each team name is unique across the league.

2. **Teams Table**: The `TeamName` is likely a candidate key here, assuming each team has a unique name and there are no two teams with the same name.

3. **Points Table**: The `TeamPosition` could be a candidate key if it uniquely identifies each row. However, since positions can change and might not be unique, `TeamName` could be a better candidate key, assuming it's unique.

In practice, a primary key is chosen from the set of candidate keys. The primary key is then used to enforce entity integrity by ensuring that each row in the table is uniquely identifiable. The other candidate keys not chosen as the primary key are often referred to as alternate keys. If a candidate key consists of more than one attribute, it is called a composite key.

For the tables you've provided, assuming that `TeamName` is unique across all tables, it would be a suitable candidate key for all three tables. However, in a real-world scenario, it's common to use a surrogate key (like an auto-incrementing ID) that doesn't have any business meaning but serves as a unique identifier for each row.


A **composite key** is a type of candidate key that consists of two or more columns, which together uniquely identify each row in a table. In your tables, a composite key would be necessary if no single column can serve as a unique identifier by itself.

Let's consider the `Points` table from your example:

```sql
CREATE TABLE Points (
    TeamPosition INT,
    TeamName VARCHAR(100),
    Points INT
);
```

If both `TeamPosition` and `TeamName` are needed to uniquely identify a row because neither alone is unique, they can be combined to form a composite key. Here's how you would define a composite key for the `Points` table:

```sql
ALTER TABLE Points
ADD PRIMARY KEY (TeamPosition, TeamName);
```

In this case, the combination of `TeamPosition` and `TeamName` must be unique across all rows in the `Points` table. This ensures that no two teams can have the same position and name simultaneously, which aligns with the real-world scenario where a team can only occupy one position in the league at a time.

Composite keys are particularly useful in association tables that represent many-to-many relationships between entities. They help maintain data integrity by preventing duplicate entries for the same combination of related entities.