# SELECT Statement

The `SELECT ` statement is the primary tool for retrieving data from database tables. It forms the core of data interaction in SQL, enabling projection operations from Relational Algebra.

Use `SELECT` to retrieve all records or filter records based on specific criteria.

First, create the necessary database and table for demonstration.

```sql
CREATE DATABASE sql_demo;
```

Switch to the database:

```sql
USE sql_demo;
```

Create a `users` table:

```sql
CREATE TABLE users
(
id INT PRIMARY KEY AUTO_INCREMENT,
username VARCHAR(255) NOT NULL,
about TEXT,
email VARCHAR(255),
birthday DATE,
active BOOL
);
```

Insert initial data:

```sql
INSERT INTO users
( username, email, active )
VALUES
('shinobi', 's@valorantdigital.com', true),
(' Java Team Six', 'd@valorantdigital.com', false),
('tony', 't@valorantdigital.com', true);
```

Output confirms data insertion:

```
Query OK, 3 rows  affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0
```

## Selecting All Columns

Retrieve all data from the `users` table using `SELECT *`:

```sql
SELECT * FROM  users;
```

This statement executes the retrieval action (`SELECT`). The asterisk (`*`) specifies all columns. The `FROM` clause indicates the source table (`users`).

Output includes all entries and all columns:

```
+ ----+---------------+-------+----------+---------+---------------------+
| id | username      | about | birthday | active | email                 |
+----+---------------+-------+----------+--------+-----------------------+
|  1 | shinobi       | NULL  | NULL     |      1 | s@valorantdigital.com |
|  2 | Java Team Six | NULL  | NULL     |      0 | d@valorantdigital.com |
|  3 | tony          | NULL  | NULL     |      1 | t@valorantdigital.com |
+----+---------------+-------+----------+--------+-----------------------+
3 rows in set (0.00 sec)
```

This retrieves all 3 users created, including currently `NULL`  `about` and `birthday` fields. Often, tables have many columns; you will frequently need to select only a subset.

## Pattern Matching

SQL pattern matching finds data without knowing the exact value. Use wildcard characters (`_`, `%`) with  `LIKE` or `ILIKE`.

*   `_`: Matches any single character.
*   `%`: Matches any number of characters (including zero).

Find usernames ending with `y`:

```sql
SELECT * FROM users WHERE username LIKE '%y';
```

Output:

```
+----+----------+-------+----------+--------+-----------------------+
| id | username | about | birthday | active | email                 |
+----+----------+-------+----------+--------+-----------------------+
|  1 | shinobi  | NULL  | NULL     |      1 | s@valorantdigital.com |
|  3 | tony     | NULL  | NULL     |      1 | t@valorantdigital.com |
+----+---------+-------+----------+--------+-----------------------+
```

The `%` before `y` matches any characters preceding `y`.

To match a specific number of characters, use multiple `_`. Each `_` is  one character.

Find usernames with `e` as the second character:

```sql
SELECT * FROM users WHERE username LIKE '_e%';
```

Output:

```
+----+---------------+-------+----------+--------+-----------------------+
| id | username      | about | birthday | active | email                 |
+----+---------------+-------+----------+--------+-----------------------+
|  2 | Java Team Six | NULL  | NULL     |      0 | d@valorantdigital.com |
+----+---------------+-------+----------+--------+-----------------------+
```

Note that `LIKE` is case-sensitive. Use `ILIKE` to perform case-insensitive pattern matching.

## Formatting Results

Each SQL statement terminates with  a semi-colon (`;`). Alternatively, use `\G` to format output as a list, improving readability for tables with numerous columns.

```sql
SELECT * FROM users \G
```

Output is formatted vertically:

```
***************************  1. row ***************************
id: 1
username: shinobi
about: NULL
birthday: NULL
active: 1
email: s@valorantdigital.com
*************************** 2 . row ***************************
id: 2
username: Java Team Six
about: NULL
birthday: NULL
active: 0
email: d@valorantdigital.com
...
```

This vertical format  is practical when table width exceeds display capacity.

## Selecting Specific Columns

Instead of `*`, list desired columns separated by commas to retrieve only those fields. Retrieve `username` and `active`:

```sql
SELECT username, active  FROM users;
```

Output contains only the specified columns:

```
+---------------+--------+
| username      | active |
+---------------+--------+
| shinobi       |      1 |
| Java Team Six |      0 |
| tony          |      1 |
+---------------+--------+
```

Note: SQL names are case-insensitive (e.g., `username` ≡ `USERNAME` ≡ `userName`).

## SELECT Without a  FROM Clause

A `SELECT` statement can retrieve literal values without referencing a table (`FROM` clause).

```sql
SELECT 'Troy' as username;
```

Output:

```
+----------+
| username |
+----------+
|  Troy    |
+----------+
```

## SELECT with Arithmetic Operations

Apply arithmetic expressions (+, –, *, /) directly within the `SELECT` clause.

Example: Multiply the `active` column value by 5 and alias  the result as `new_active`.

```sql
SELECT username, active*5 as new_active FROM users;
```

Output:

```
+---------------+------------+
| username      | new_active |
+---------------+------------+
| shinobi       |          5 |
| Java Team Six |          0 |
| tony          |          5 |
+---------------+------------+
```

## Limiting Results

The `LIMIT` clause restricts the number of rows returned. Retrieve  only 1 user:

```sql
SELECT * FROM users LIMIT 1;
```

Output provides a single row:

```
+----+----------+-------+----------+--------+-----------------------+
| id | username | about | birthday | active | email                 |
+----+----------+-------+----------+--------+-----------------------+
|  1 | shinobi  | NULL  | NULL     |      1 | s@valorantdigital.com |
+----+----------+-------+----------+--------+-----------------------+
```

Modify the number following `LIMIT` to retrieve more rows (e.g., `LIMIT 2`).

## Aggregate Functions (COUNT, MIN, MAX, AVG, SUM)

Aggregate functions perform  calculations across a set of rows.

*   `COUNT`: Returns the number of rows.

```sql
SELECT COUNT(*) FROM users;
```

Output:

```
+----------+
| COUNT(*) |
+----------+
|        3 |
+----------+
```

*   `MIN`: Returns the smallest value in a column. Get the lowest user ID:

```sql
SELECT MIN(id) FROM users ;
```

This returns `1`.

*   `MAX`: Returns the largest value in a column. Get the highest user ID:

```sql
SELECT MAX(id) FROM users;
```

This returns `3`.

*   `AVG`: Returns the average value of a numeric column. Calculate the average ID:
```sql
SELECT AVG(id) FROM users;
```

(1 + 2 +  3) / 3 results in `2`.

*   `SUM`: Returns the total sum of values in a numeric column. Sum the IDs:

```sql
SELECT SUM(id) FROM users;
```

1 + 2 + 3 results in `6`.

## Retrieving Distinct Values

The `DISTINCT` keyword eliminates duplicate rows from the result set.

Insert duplicate data for demonstration:

```sql
INSERT INTO users
(  username, email, active )
VALUES
('shinobi', 's@valorantdigital.com', true),
('Java Team Six', 'd@valorantdigital.com', false),
('tony ', 't@valorantdigital.com', true);
```

The `users` table now has 6 entries, with usernames repeated.

Select all usernames (including duplicates):

```sql
SELECT username FROM users;
```

Output shows repetitions:

```
+---------------+
| username      |
+---------------+
| shinobi       |
| Java Team Six |
| tony          |
| shinobi       |
| Java Team Six |
| tony          |
+ ---------------+
```

Use `DISTINCT` to show only unique usernames:

```sql
SELECT DISTINCT username FROM users;
```

Output contains only unique entries:

```
+---------------+
| username      |
+---------------+
| shinobi       |
| Java Team Six |
| tony          |
+---------------+
```

The duplicate usernames are removed from the result set.