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

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2022-07-18 00:24:02 +01:00
# SQL Primer
## Agenda
* CRUD with SQL
* JOIN
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> **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).
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## 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;
```