# 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).

## SQL Commands

![SQL](https://i.stack.imgur.com/7uUaJ.png)

### DDL

DDL is short name of Data Definition Language, which deals with database schemas and descriptions, of how the data should reside in the database.

* **CREATE** – to create database and its objects like (table, index, views, store procedure, function and triggers).
* **ALTER** – alters the structure of the existing database.
* **DROP** – delete objects from the database.
* **TRUNCATE** – remove all records from a table; also, all spaces allocated for the records are removed.
* **COMMENT** – add comments to the data dictionary.
* **RENAME** – rename an object.

### DML

DML is short name of Data Manipulation Language which deals with data manipulation, and includes most common SQL statements such SELECT, INSERT, UPDATE, DELETE etc, and it is used to store, modify, retrieve, delete and update data in database.

* **SELECT** – retrieve data from one or more tables.
* **INSERT** – insert data into a table.
* **UPDATE** – updates existing data within a table.
* **DELETE** – delete all records from a table.
* **MERGE** – UPSERT operation (insert or update)
* **CALL** – call a PL/SQL or Java subprogram.
* **EXPLAIN PLAN** – interpretation of the data access path.
* **LOCK TABLE** – concurrency control.

### DCL

DCL is short name of Data Control Language which includes commands such as GRANT, and mostly concerned with rights, permissions and other controls of the database system.

* **GRANT** – allow users access privileges to database.
* **REVOKE** – withdraw users access privileges given by using the GRANT command.

### TCL

TCL is short name of Transaction Control Language which deals with transaction within a database.

* **COMMIT** – commits a transaction.
* **ROLLBACK** – rollback a transaction in case of any error occurs.
* **SAVEPOINT** – a point inside a transaction that allows rollback state to what it was at the time of the savepoint.
* **SET TRANSACTION** – specify characteristics for the transaction.

## 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);
     ```

## Joins

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;
```