notebook/IT/SQL/Cheasheet.md

# SQL Cheatsheet

## Quick SQL Cheatsheet

A quick reminder of all relevant SQL queries and examples on how to use them.

This repository is constantly being updated and added to by the community.
Pull requests are welcome. Enjoy!

## Table of Contents

1. [Finding Data Queries.](#find)
2. [Data Modification Queries.](#modify)
3. [Reporting Queries.](#report)
4. [Join Queries.](#joins)
5. [View Queries.](#view)
6. [Altering Table Queries.](#alter)
7. [Creating Table Query.](#create)

<a name="find"></a>

## 1. Finding Data Queries

### **SELECT**: used to select data from a database

* `SELECT` * `FROM` table_name;

### **DISTINCT**: filters away duplicate values and returns rows of specified column

* `SELECT DISTINCT` column_name;

### **WHERE**: used to filter records/rows

* `SELECT` column1, column2 `FROM` table_name `WHERE` condition;
* `SELECT` * `FROM` table_name `WHERE` condition1 `AND` condition2;
* `SELECT` * `FROM` table_name `WHERE` condition1 `OR` condition2;
* `SELECT` * `FROM` table_name `WHERE NOT` condition;
* `SELECT` * `FROM` table_name `WHERE` condition1 `AND` (condition2 `OR` condition3);
* `SELECT` * `FROM` table_name `WHERE EXISTS` (`SELECT` column_name `FROM` table_name `WHERE` condition);

### **ORDER BY**: used to sort the result-set in ascending or descending order

* `SELECT` * `FROM` table_name `ORDER BY` column;
* `SELECT` * `FROM` table_name `ORDER BY` column `DESC`;
* `SELECT` * `FROM` table_name `ORDER BY` column1 `ASC`, column2 `DESC`;

### **SELECT TOP**: used to specify the number of records to return from top of table

* `SELECT TOP` number columns_names `FROM` table_name `WHERE` condition;
* `SELECT TOP` percent columns_names `FROM` table_name `WHERE` condition;
* Not all database systems support `SELECT TOP`. The MySQL equivalent is the `LIMIT` clause
* `SELECT` column_names `FROM` table_name `LIMIT` offset, count;

### **LIKE**: operator used in a WHERE clause to search for a specific pattern in a column

* % (percent sign) is a wildcard character that represents zero, one, or multiple characters
* _ (underscore) is a wildcard character that represents a single character
* `SELECT` column_names `FROM` table_name `WHERE` column_name `LIKE` pattern;
* `LIKE` ‘a%’ (find any values that start with “a”)
* `LIKE` ‘%a’ (find any values that end with “a”)
* `LIKE` ‘%or%’ (find any values that have “or” in any position)
* `LIKE` ‘_r%’ (find any values that have “r” in the second position)
* `LIKE` ‘a_%_%’ (find any values that start with “a” and are at least 3 characters in length)
* `LIKE` ‘[a-c]%’ (find any values starting with “a”, “b”, or “c”

### **IN**: operator that allows you to specify multiple values in a WHERE clause

* essentially the IN operator is shorthand for multiple OR conditions
* `SELECT` column_names `FROM` table_name `WHERE` column_name `IN` (value1, value2, …);
* `SELECT` column_names `FROM` table_name `WHERE` column_name `IN` (`SELECT STATEMENT`);

### **BETWEEN**: operator selects values within a given range inclusive

* `SELECT` column_names `FROM` table_name `WHERE` column_name `BETWEEN` value1 `AND` value2;
* `SELECT` * `FROM` Products `WHERE` (column_name `BETWEEN` value1 `AND` value2) `AND NOT` column_name2 `IN` (value3, value4);
* `SELECT` * `FROM` Products `WHERE` column_name `BETWEEN` #01/07/1999# AND #03/12/1999#;

### **NULL**: values in a field with no value

* `SELECT` * `FROM` table_name `WHERE` column_name `IS NULL`;
* `SELECT` * `FROM` table_name `WHERE` column_name `IS NOT NULL`;

### **AS**: aliases are used to assign a temporary name to a table or column

* `SELECT` column_name `AS` alias_name `FROM` table_name;
* `SELECT` column_name `FROM` table_name `AS` alias_name;
* `SELECT` column_name `AS` alias_name1, column_name2 `AS` alias_name2;
* `SELECT` column_name1, column_name2 + ‘, ‘ + column_name3 `AS` alias_name;

### **UNION**: set operator used to combine the result-set of two or more SELECT statements

* Each SELECT statement within UNION must have the same number of columns
* The columns must have similar data types
* The columns in each SELECT statement must also be in the same order
* `SELECT` columns_names `FROM` table1 `UNION SELECT` column_name `FROM` table2;
* `UNION` operator only selects distinct values, `UNION ALL` will allow duplicates

### **INTERSECT**: set operator which is used to return the records that two SELECT statements have in common

* Generally used the same way as **UNION** above
* `SELECT` columns_names `FROM` table1 `INTERSECT SELECT` column_name `FROM` table2;

### **EXCEPT**: set operator used to return all the records in the first SELECT statement that are not found in the second SELECT statement

* Generally used the same way as **UNION** above
* `SELECT` columns_names `FROM` table1 `EXCEPT SELECT` column_name `FROM` table2;

### **ANY|ALL**: operator used to check subquery conditions used within a WHERE or HAVING clauses

* The `ANY` operator returns true if any subquery values meet the condition
* The `ALL` operator returns true if all subquery values meet the condition
* `SELECT` columns_names `FROM` table1 `WHERE` column_name operator (`ANY`|`ALL`) (`SELECT` column_name `FROM` table_name `WHERE` condition);

### **GROUP BY**: statement often used with aggregate functions (COUNT, MAX, MIN, SUM, AVG) to group the result-set by one or more columns

* `SELECT` column_name1, COUNT(column_name2) `FROM` table_name `WHERE` condition `GROUP BY` column_name1 `ORDER BY` COUNT(column_name2) DESC;

### **HAVING**: this clause was added to SQL because the WHERE keyword could not be used with aggregate functions

* `SELECT` `COUNT`(column_name1), column_name2 `FROM` table `GROUP BY` column_name2 `HAVING` `COUNT(`column_name1`)` > 5;

### **WITH**: often used for retrieving hierarchical data or re-using temp result set several times in a query. Also referred to as "Common Table Expression"

* `WITH RECURSIVE` cte `AS` (<br/>
    &nbsp;&nbsp;`SELECT` c0.\* `FROM` categories `AS` c0 `WHERE` id = 1 `# Starting point`<br/>
    &nbsp;&nbsp;`UNION ALL`<br/>
    &nbsp;&nbsp;`SELECT` c1.\* `FROM` categories `AS` c1 `JOIN` cte `ON` c1.parent_category_id = cte.id<br/>
  )<br/>
  `SELECT` *<br/>
  `FROM` cte

<a name="modify"></a>

## 2. Data Modification Queries

### **INSERT INTO**: used to insert new records/rows in a table

* `INSERT INTO` table_name (column1, column2) `VALUES` (value1, value2);
* `INSERT INTO` table_name `VALUES` (value1, value2 …);

### **UPDATE**: used to modify the existing records in a table

* `UPDATE` table_name `SET` column1 = value1, column2 = value2 `WHERE` condition;
* `UPDATE` table_name `SET` column_name = value;

### **DELETE**: used to delete existing records/rows in a table

* `DELETE FROM` table_name `WHERE` condition;
* `DELETE` * `FROM` table_name;

<a name="report"></a>

## 3. Reporting Queries

### **COUNT**: returns the # of occurrences

* `SELECT COUNT (DISTINCT` column_name`)`;

### **MIN() and MAX()**: returns the smallest/largest value of the selected column

* `SELECT MIN (`column_names`) FROM` table_name `WHERE` condition;
* `SELECT MAX (`column_names`) FROM` table_name `WHERE` condition;

### **AVG()**: returns the average value of a numeric column

* `SELECT AVG (`column_name`) FROM` table_name `WHERE` condition;

### **SUM()**: returns the total sum of a numeric column

* `SELECT SUM (`column_name`) FROM` table_name `WHERE` condition;

<a name="joins"></a>

## 4. Join Queries

### **INNER JOIN**: returns records that have matching value in both tables

* `SELECT` column_names `FROM` table1 `INNER JOIN` table2 `ON` table1.column_name=table2.column_name;
* `SELECT` table1.column_name1, table2.column_name2, table3.column_name3 `FROM` ((table1 `INNER JOIN` table2 `ON` relationship) `INNER JOIN` table3 `ON` relationship);

### **LEFT (OUTER) JOIN**: returns all records from the left table (table1), and the matched records from the right table (table2)

* `SELECT` column_names `FROM` table1 `LEFT JOIN` table2 `ON` table1.column_name=table2.column_name;

### **RIGHT (OUTER) JOIN**: returns all records from the right table (table2), and the matched records from the left table (table1)

* `SELECT` column_names `FROM` table1 `RIGHT JOIN` table2 `ON` table1.column_name=table2.column_name;

### **FULL (OUTER) JOIN**: returns all records when there is a match in either left or right table

* `SELECT` column_names `FROM` table1 ``FULL OUTER JOIN`` table2 `ON` table1.column_name=table2.column_name;

### **Self JOIN**: a regular join, but the table is joined with itself

* `SELECT` column_names `FROM` table1 T1, table1 T2 `WHERE` condition;

<a name="view"></a>

## 5. View Queries

### **CREATE**: create a view

* `CREATE VIEW` view_name `AS SELECT` column1, column2 `FROM` table_name `WHERE` condition;

### **SELECT**: retrieve a view

* `SELECT` * `FROM` view_name;

### **DROP**: drop a view

* `DROP VIEW` view_name;

<a name="alter"></a>

## 6. Altering Table Queries

### **ADD**: add a column

* `ALTER TABLE` table_name `ADD` column_name column_definition;

### **MODIFY**: change data type of column

* `ALTER TABLE` table_name `MODIFY` column_name column_type;

### **DROP**: delete a column

* `ALTER TABLE` table_name `DROP COLUMN` column_name;

<a name="create"></a>

## 7. Creating Table Query

### **CREATE**: create a table

* `CREATE TABLE` table_name `(` <br />
   `column1` `datatype`, <br />
   `column2` `datatype`, <br />
   `column3` `datatype`, <br />
   `column4` `datatype`, <br />
   `);`
   :q
   :q