# [Github cheatsheet](https://github.com/khanhnamle1994/cracking-the-data-science-interview/tree/master/Cheatsheets#sql)

## SQL

* [Finding Data Queries](#finding-data-queries)
* [Data Modification Queries](#data-modification-queries)
* [Reporting Queries](#reporting-queries)
* [Join Queries](#join-queries)
* [View Queries](#view-queries)
* [Altering Table Queries](#altering-table-queries)
* [Creating Table Query](#creating-table-query)

### 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` ‘[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;

### 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;

### 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;

### 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;

### 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;

### 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;

### Creating Table Query

### **CREATE**: create a table
* `CREATE TABLE` table_name `(` <br />
   `column1` `datatype`, <br />
   `column2` `datatype`, <br />
   `column3` `datatype`, <br />
   `column4` `datatype`, <br />
   `);`


