fundamentals/database/notes/05-sql-primer.md

# SQL Primer

## Agenda
* CRUD with SQL
* JOIN


> **Note**
>
> The script to create the database and insert data can be found [here](../code/create_database.sql). \
> Once your database is set up, download the worksheet from [here](../notes/05-sql-primer-worksheet.md) and start writing some queries!
> If you get stuck, you can find the answers [here](../notes/05-sql-primer-worksheet-answers.md).

## CRUD with SQL

### Create rows

**Keyword**: `INSERT`
**Syntax**: `INSERT INTO table_name (column1, column2, ...) VALUES (value1, value2, ...)`

#### Examples

1. Insert a row with all columns
```sql
    INSERT INTO students VALUES (1, 'Tantia', 'Tope', 't@t.com', '1234567890', 1);
```

2. Insert a row with some columns
```sql
    INSERT INTO students (first_name, last_name) VALUES ('Tantia', 'Tope');
```
### Read rows

**Keyword**: `SELECT`
**Syntax**: `SELECT column1, column2, ... FROM table_name WHERE condition ORDER BY column1, column2, ... ASC/DESC LIMIT #`

#### Examples

1. Get all rows

    ```sql
    SELECT * FROM students;
    ```

2. Get certain fields from all rows

    ```sql
    SELECT first_name, last_name FROM students;
    ```
3. Filter rows by condition
    
    ```sql
    SELECT * FROM students WHERE first_name = 'Tantia';
    ```
4. Order rows by column
    
    ```sql
    SELECT * FROM students ORDER BY first_name ASC;
    ```
5. Limit number of rows
    
    ```sql
    SELECT * FROM students LIMIT 10;
    ```

#### Common operators for WHERE clause

| Operator | Description           | Example                                                                     |
| -------- | --------------------- | --------------------------------------------------------------------------- |
| =        | Equal                 | `SELECT * FROM students WHERE first_name = 'Tantia'`                        |
| != or <> | Not equal             | `SELECT * FROM students WHERE first_name != 'Tantia'`                       |
| NOT      | NOT                   | `SELECT * FROM students WHERE NOT first_name = 'John'`                      |
| >        | Greater than          | `SELECT * FROM students WHERE iq > 150`                                     |
| <        | Less than             | `SELECT * FROM students WHERE age < 100`                                    |
| >=       | Greater than or equal | `SELECT * FROM students WHERE age >= 18`                                    |
| <=       | Less than or equal    | `SELECT * FROM students WHERE age <= 18`                                    |
| AND      | AND                   | `SELECT * FROM students WHERE first_name = 'Tantia' AND last_name = 'Tope'` |
| OR       | OR                    | `SELECT * FROM students WHERE first_name = 'John' OR last_name = 'Mycroft'` |
| IN       | IN                    | `SELECT * FROM students WHERE first_name IN ('John', 'Mycroft')`            |
| BETWEEN  | BETWEEN               | `SELECT * FROM students WHERE iq BETWEEN 100 AND 150`                       |
| LIKE     | LIKE                  | `SELECT * FROM students WHERE first_name LIKE '%T%'`                        |
| REGEXP   | REGEXP                | `SELECT * FROM students WHERE first_name REGEXP '^[A-Z]{1}'`                |
| NULL     | NULL                  | `SELECT * FROM students WHERE first_name IS NULL`                           |
| NOT NULL | NOT NULL              | `SELECT * FROM students WHERE first_name IS NOT NULL`                       |

#### String matching wildcards

With `LIKE` you can use the following two wildcard characters in the pattern:
* `%` matches any number of characters, even zero characters.
* `_` matches exactly one character.

```sql
SELECT * FROM students WHERE first_name LIKE 'T%';
```

```sql
SELECT * FROM students WHERE first_name LIKE 'T_';
```

### Update rows

**Keyword**: `UPDATE`
**Syntax**: `UPDATE table_name SET column1 = value1, column2 = value2, ... WHERE condition`
    
##### Examples
    
  1. Update a row
  
      ```sql
      UPDATE students SET first_name = 'Tantia' WHERE id = 1;
      ```
  2. Update a row with a condition
  
      ```sql
      UPDATE students SET first_name = 'Tantia' WHERE id = 1 AND first_name = 'John';
      ```
  
  3. Update multiple columns
  
      ```sql
      UPDATE students SET first_name = 'Tantia', last_name = 'Tope' WHERE id = 1 AND first_name = 'John';
      ```

### Delete rows

**Keyword**: `DELETE`
**Syntax**: `DELETE FROM table_name WHERE condition`
    
##### Examples
    

1. Delete a row with a condition
 
     ```sql
     DELETE FROM students WHERE id = 1 AND first_name = 'John';
     ```
 
2. Delete a multiple rows
 
     ```sql
     DELETE FROM students WHERE id IN (1, 2, 3);
     ```

## Join

Join is the widely-used clause in the SQL Server essentially to combine and retrieve data from two or more tables. In a real-world relational database, data is structured in many tables and which is why, there is a constant need to join these multiple tables based on logical relationships between them.

### Inner join

Inner Join clause in SQL Server creates a new table (not physical) by combining rows that have matching values in two or more tables. This join is based on a logical relationship (or a common field) between the tables and is used to retrieve data that appears in both tables.

![Inner Join](https://i.stack.imgur.com/kZcvR.gif)

Assume, we have two tables, Table A and Table B, that we would like to join using SQL Inner Join. The result of this join will be a new result set that returns matching rows in both these tables. The intersection part in black below shows the data retrieved using Inner Join.

![Inner Join](https://www.sqlshack.com/wp-content/uploads/2019/06/venn-diagram-representation-of-sql-inner-join-.png)

**Keyword**: `INNER JOIN` or simply `JOIN`
**Syntax**: `SELECT column1, column2, ... FROM table_name1 JOIN table_name2 ON condition`

For example, we want to get the batch names of all the students along with their names.
| id  | first_name | last_name | batch_name |
| --- | ---------- | --------- | ---------- |
| 1   | John       | Watson    | Sherlock   |
| 2   | Mycroft    | Holmes    | Sherlock   |

This can be achieved by using the following SQL query:

```sql
SELECT s.first_name, s.last_name, b.batch_name FROM students s JOIN batches ON s.batch_id = b.id;
```