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database/notes/05-sql-primer.md
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DELETE FROM students WHERE id IN (1, 2, 3); DELETE FROM students WHERE id IN (1, 2, 3);
``` ```
## Join ## 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. 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.

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database/notes/06-sql-joins-aggregation.md Normal file
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# SQL Joins and Aggregation
## Agenda
* SQL Joins
* Aggregation Functions (SUM, AVG, COUNT etc)
* Sub-queries
* Built-in Functions
## 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;
```
### Left Outer Join
Let us consider the students and batches tables below:
| id | first_name | last_name | batch_id |
| --- | ---------- | --------- | -------- |
| 1 | John | Watson | 1 |
| 2 | Mycroft | Holmes | 1 |
| 3 | Moriarty | Patel | NULL |
| id | batch_name |
| --- | ---------------- |
| 1 | Sherlock Academy |
| 2 | Crime Academy |
Now if we decide to get the batch names of all the students along with their names with an `inner join` we will get the following result:
| id | first_name | last_name | batch_name |
| --- | ---------- | --------- | ---------------- |
| 1 | John | Watson | Sherlock Academy |
| 2 | Mycroft | Holmes | Sherlock Academy |
Since both John and Mycroft had a valid batch_id, the respective rows were merged to produce the result.
But Moriarty did not have a valid batch_id, so the result set will not contain him.
But if we want to run a query to get all students along with their batch names or NULL if they do not have a batch_id, we can use the **Left Outer Join**.
> The LEFT JOIN keyword returns all records from the left table (table1), and the matching records from the right table (table2). The result is 0 records from the right side, if there is no match.
![Left Outer Join](https://www.w3schools.com/sql/img_leftjoin.gif)
**Keyword**: `LEFT OUTER JOIN` or simply `LEFT JOIN`
**Syntax**: `SELECT column1, column2, ... FROM table_name1 LEFT JOIN table_name2 ON condition`
The query for fetching students with their batch names now will be:
```sql
SELECT
s.first_name, s.last_name, b.name
FROM
students s
LEFT JOIN
batches b ON s.batch_id = b.id;
```
And the result will be:
| id | first_name | last_name | batch_name |
| --- | ---------- | --------- | ---------------- |
| 1 | John | Watson | Sherlock Academy |
| 2 | Mycroft | Holmes | Sherlock Academy |
| 3 | Moriarty | Patel | NULL |
### Right Outer Join
Similarly, `RIGHT OUTER JOIN` returns all records from the right table (table2), and the matching records from the left table (table1). The result is 0 records from the left side, if there is no match.
Now if we want to get all the batches along with their students if present else `NULL`, we can use `RIGHT OUTER JOIN`.
**Keyword**: `RIGHT OUTER JOIN` or simply `RIGHT JOIN`
**Syntax**: `SELECT column1, column2, ... FROM table_name1 RIGHT JOIN table_name2 ON condition`
![Right Join](https://www.w3schools.com/sql/img_rightjoin.gif)
The query for fetching batches with their students now will be:
```sql
SELECT
s.first_name, s.last_name, b.name
FROM
students s
RIGHT JOIN
batches b ON s.batch_id = b.id;
```
This will return the following result:
| id | first_name | last_name | batch_name |
| ---- | ---------- | --------- | ---------------- |
| 1 | John | Watson | Sherlock Academy |
| 2 | Mycroft | Holmes | Sherlock Academy |
| NULL | NULL | NULL | Crime Academy |
### Full Outer Join
The FULL OUTER JOIN clause returns a result set that includes rows from both left and right tables.
When no matching rows exist for the row in the left table, the columns of the right table will contain NULL. Likewise, when no matching rows exist for the row in the right table, the column of the left table will contain NULL.
![Full Outer Join](https://www.w3schools.com/sql/img_fulljoin.gif)
**Keyword**: `FULL OUTER JOIN` or simply `FULL JOIN`
**Syntax**: `SELECT column1, column2, ... FROM table_name1 FULL JOIN table_name2 ON condition`
The query for fetching batches with their students now will be:
```sql
SELECT
s.first_name, s.last_name, b.name
FROM
students s
FULL JOIN
batches b ON s.batch_id = b.id;
```
This will return the following result:
| id | first_name | last_name | batch_name |
| ---- | ---------- | --------- | ---------------- |
| 1 | John | Watson | Sherlock Academy |
| 2 | Mycroft | Holmes | Sherlock Academy |
| 3 | Moriarty | Patel | NULL |
| NULL | NULL | NULL | Crime Academy |
> **Warning**
> MySQL does not support the FULL OUTER JOIN clause.
### Cross Join
The result set from a `cross join` will include all rows from both tables, where each row is the combination of the row in the first table with the row in the second table. In general, if each table has `n` and `m` rows respectively, the result set will have `n` x `m` rows.
In other words, the CROSS JOIN clause returns a Cartesian product of rows from the joined tables.
**Keyword**: `CROSS JOIN`
**Syntax**: `SELECT column1, column2, ... FROM table_name1 CROSS JOIN table_name2`
**Syntax 2**: `SELECT column1, column2, ... FROM table_name1, table_name2`
An example query for fetching from the students and batches tables will be:
```sql
SELECT
s.first_name, s.last_name, b.name
FROM
students s
CROSS JOIN
batches b;
```
Note that different from the INNER JOIN, LEFT JOIN , and RIGHT JOIN clauses, the CROSS JOIN clause does not have a join predicate. In other words, it does not have the ON or USING clause.
If you add a WHERE clause, in case table t1 and t2 has a relationship, the CROSS JOIN works like the INNER JOIN.
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