SQL Data Query Language (DQL)

## SQL Data Query Language (DQL)
---
## DQL Statement
* SELECT
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## Relational Algebra
* Set operations (union, difference, Cartesian product)
* Projection
* Selection
* Aggregation
* Join
* Rename
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## Projection
* presents a subset of the attributes of a relation
* depending on the data, may produce duplicate rows
* duplicates, can be eliminated using the `DISTINCT` keyword
* `*` represents all the attributes
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## Projection
```sql 
SELECT first_name, gender
FROM employees;
```
---
## Distinct Projection
```sql 
SELECT DISTINCT first_name, gender
FROM employees;
```
---
## Selection
* presents a subset of tuples of a relation 
* uses `WHERE` clause to specify criteria
* often used in conjunction with a projection operation
---
## Selection
```sql 
SELECT first_name, last_name
FROM employees 
WHERE birth_date > "1960-01-01"
```
---
## Aggregation
* computes a function on a column
* count, sum, average, minimum, maximum
* can be calculated over entire table, or distinct groups of attributes
* often used in conjuntion with selection to work on a subset of rows
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## Aggregation 
```sql 
SELECT MAX(salary) 
FROM salaries; 
```
---
## Aggregation 
```sql 
SELECT MIN(salary) 
FROM salaries
WHERE from_date LIKE "1986%"; 
```
---
## Aggregation with Grouping
```sql 
SELECT MAX(salary) 
FROM salaries
GROUP BY YEAR(from_date); 
```
---
## Aggregation with Grouping
```sql 
SELECT YEAR(from_date), MAX(salary)
FROM salaries
GROUP BY YEAR(from_date); 
```
---
## Aggregation with Grouping
```sql 
SELECT YEAR(from_date), MAX(salary), COUNT(*) 
FROM salaries
GROUP BY YEAR(from_date); 
```
---
## Joins
* Inner
* Left Outer
* Right Outer