sql cheatsheet and basic commands
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# 14. Databases
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## Basic SQL
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```sql
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SELECT CITY FROM STATION WHERE CODE = '123'
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```
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Get distinct cities:
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```sql
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SELECT DISTINCT CITY FROM STATION WHERE CODE = '123'
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```
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Count number of distinct cities
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```sql
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SELECT count(DISTINCT CITY) FROM STATION WHERE CODE = '123'
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```
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Apart from count, there is also `sum`, `avg`, `ceil`, `floor`.
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Get the shortest city name and longest city name
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```sql
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SELECT CITY, LENGTH(CITY) FROM STATION
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ORDER BY LENGTH(CITY), CITY asc
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limit 1;
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SELECT CITY, LENGTH(CITY) FROM STATION
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ORDER BY LENGTH(CITY), CITY desc
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limit 1;
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```
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Get cities ending with vowels
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```sql
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SELECT DISTINCT CITY FROM STATION WHERE city REGEXP "[aeiou]$"
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```
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Select substring of column. The first two characters. 1 means first character, 2 means length. SQL has an index system starting from 1. To get the last ones, use -X.
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```sql
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SELECT SUBSTR(NAME, 1, 2) FROM TABLE
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```
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Output a string for each case
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```sql
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SELECT
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CASE
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WHEN A >= (B + C) OR B >= (A + C) OR C >= (A + B) THEN 'Not A Triangle'
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WHEN A = B AND A = C THEN 'Equilateral'
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WHEN A = B OR B = C OR A = C THEN 'Isosceles'
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ELSE 'Scalene'
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END
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FROM TRIANGLES;
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```
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Get the list of top earning employees, and the number of employees that earn this amount
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```sql
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SELECT earnings, count(*) FROM Employee
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GROUP BY earnings
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ORDER BY earnings desc
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limit 1;
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```
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```sql
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SELECT COUNTRY.Continent, FLOOR(AVG(CITY.Population))
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FROM CITY INNER JOIN COUNTRY
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ON CITY.CountryCode=COUNTRY.Code
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GROUP BY COUNTRY.Continent;
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```
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## Book SQL
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```sql
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SELECT CourseName, TeacherName FROM Courses, Teachers WHERE Courses.TeacherID = Teachers.TeacherID
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```
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@ -0,0 +1,174 @@
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# [Github cheatsheet](https://github.com/khanhnamle1994/cracking-the-data-science-interview/tree/master/Cheatsheets#sql)
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## SQL
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* [Finding Data Queries](#finding-data-queries)
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* [Data Modification Queries](#data-modification-queries)
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* [Reporting Queries](#reporting-queries)
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* [Join Queries](#join-queries)
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* [View Queries](#view-queries)
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* [Altering Table Queries](#altering-table-queries)
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* [Creating Table Query](#creating-table-query)
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### Finding Data Queries
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#### **SELECT**: used to select data from a database
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* `SELECT` * `FROM` table_name;
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#### **DISTINCT**: filters away duplicate values and returns rows of specified column
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* `SELECT DISTINCT` column_name;
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#### **WHERE**: used to filter records/rows
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* `SELECT` column1, column2 `FROM` table_name `WHERE` condition;
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* `SELECT` * `FROM` table_name `WHERE` condition1 `AND` condition2;
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* `SELECT` * `FROM` table_name `WHERE` condition1 `OR` condition2;
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* `SELECT` * `FROM` table_name `WHERE NOT` condition;
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* `SELECT` * `FROM` table_name `WHERE` condition1 `AND` (condition2 `OR` condition3);
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* `SELECT` * `FROM` table_name `WHERE EXISTS` (`SELECT` column_name `FROM` table_name `WHERE` condition);
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#### **ORDER BY**: used to sort the result-set in ascending or descending order
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* `SELECT` * `FROM` table_name `ORDER BY` column;
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* `SELECT` * `FROM` table_name `ORDER BY` column `DESC`;
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* `SELECT` * `FROM` table_name `ORDER BY` column1 `ASC`, column2 `DESC`;
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#### **SELECT TOP**: used to specify the number of records to return from top of table
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* `SELECT TOP` number columns_names `FROM` table_name `WHERE` condition;
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* `SELECT TOP` percent columns_names `FROM` table_name `WHERE` condition;
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* Not all database systems support `SELECT TOP`. The MySQL equivalent is the `LIMIT` clause
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* `SELECT` column_names `FROM` table_name `LIMIT` offset, count;
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#### **LIKE**: operator used in a WHERE clause to search for a specific pattern in a column
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* % (percent sign) is a wildcard character that represents zero, one, or multiple characters
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* _ (underscore) is a wildcard character that represents a single character
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* `SELECT` column_names `FROM` table_name `WHERE` column_name `LIKE` pattern;
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* `LIKE` ‘a%’ (find any values that start with “a”)
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* `LIKE` ‘%a’ (find any values that end with “a”)
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* `LIKE` ‘%or%’ (find any values that have “or” in any position)
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* `LIKE` ‘[a-c]%’ (find any values starting with “a”, “b”, or “c”
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#### **IN**: operator that allows you to specify multiple values in a WHERE clause
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* essentially the IN operator is shorthand for multiple OR conditions
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* `SELECT` column_names `FROM` table_name `WHERE` column_name `IN` (value1, value2, …);
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* `SELECT` column_names `FROM` table_name `WHERE` column_name `IN` (`SELECT STATEMENT`);
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#### **BETWEEN**: operator selects values within a given range inclusive
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* `SELECT` column_names `FROM` table_name `WHERE` column_name `BETWEEN` value1 `AND` value2;
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* `SELECT` * `FROM` Products `WHERE` (column_name `BETWEEN` value1 `AND` value2) `AND NOT` column_name2 `IN` (value3, value4);
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* `SELECT` * `FROM` Products `WHERE` column_name `BETWEEN` #01/07/1999# AND #03/12/1999#;
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#### **NULL**: values in a field with no value
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* `SELECT` * `FROM` table_name `WHERE` column_name `IS NULL`;
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* `SELECT` * `FROM` table_name `WHERE` column_name `IS NOT NULL`;
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#### **AS**: aliases are used to assign a temporary name to a table or column
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* `SELECT` column_name `AS` alias_name `FROM` table_name;
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* `SELECT` column_name `FROM` table_name `AS` alias_name;
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* `SELECT` column_name `AS` alias_name1, column_name2 `AS` alias_name2;
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* `SELECT` column_name1, column_name2 + ‘, ‘ + column_name3 `AS` alias_name;
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#### **UNION**: set operator used to combine the result-set of two or more SELECT statements
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* Each SELECT statement within UNION must have the same number of columns
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* The columns must have similar data types
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* The columns in each SELECT statement must also be in the same order
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* `SELECT` columns_names `FROM` table1 `UNION SELECT` column_name `FROM` table2;
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* `UNION` operator only selects distinct values, `UNION ALL` will allow duplicates
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#### **INTERSECT**: set operator which is used to return the records that two SELECT statements have in common
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* Generally used the same way as **UNION** above
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* `SELECT` columns_names `FROM` table1 `INTERSECT SELECT` column_name `FROM` table2;
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#### **EXCEPT**: set operator used to return all the records in the first SELECT statement that are not found in the second SELECT statement
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* Generally used the same way as **UNION** above
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* `SELECT` columns_names `FROM` table1 `EXCEPT SELECT` column_name `FROM` table2;
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#### **ANY|ALL**: operator used to check subquery conditions used within a WHERE or HAVING clauses
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* The `ANY` operator returns true if any subquery values meet the condition
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* The `ALL` operator returns true if all subquery values meet the condition
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* `SELECT` columns_names `FROM` table1 `WHERE` column_name operator (`ANY`|`ALL`) (`SELECT` column_name `FROM` table_name `WHERE` condition);
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#### **GROUP BY**: statement often used with aggregate functions (COUNT, MAX, MIN, SUM, AVG) to group the result-set by one or more columns
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* `SELECT` column_name1, COUNT(column_name2) `FROM` table_name `WHERE` condition `GROUP BY` column_name1 `ORDER BY` COUNT(column_name2) DESC;
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#### **HAVING**: this clause was added to SQL because the WHERE keyword could not be used with aggregate functions
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* `SELECT` `COUNT`(column_name1), column_name2 `FROM` table `GROUP BY` column_name2 `HAVING` `COUNT(`column_name1`)` > 5;
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### Data Modification Queries
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#### **INSERT INTO**: used to insert new records/rows in a table
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* `INSERT INTO` table_name (column1, column2) `VALUES` (value1, value2);
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* `INSERT INTO` table_name `VALUES` (value1, value2 …);
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#### **UPDATE**: used to modify the existing records in a table
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* `UPDATE` table_name `SET` column1 = value1, column2 = value2 `WHERE` condition;
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* `UPDATE` table_name `SET` column_name = value;
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#### **DELETE**: used to delete existing records/rows in a table
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* `DELETE FROM` table_name `WHERE` condition;
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* `DELETE` * `FROM` table_name;
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### Reporting Queries
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#### **COUNT**: returns the # of occurrences
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* `SELECT COUNT (DISTINCT` column_name`)`;
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#### **MIN() and MAX()**: returns the smallest/largest value of the selected column
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* `SELECT MIN (`column_names`) FROM` table_name `WHERE` condition;
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* `SELECT MAX (`column_names`) FROM` table_name `WHERE` condition;
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#### **AVG()**: returns the average value of a numeric column
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* `SELECT AVG (`column_name`) FROM` table_name `WHERE` condition;
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#### **SUM()**: returns the total sum of a numeric column
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* `SELECT SUM (`column_name`) FROM` table_name `WHERE` condition;
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### Join Queries
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#### **INNER JOIN**: returns records that have matching value in both tables
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* `SELECT` column_names `FROM` table1 `INNER JOIN` table2 `ON` table1.column_name=table2.column_name;
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* `SELECT` table1.column_name1, table2.column_name2, table3.column_name3 `FROM` ((table1 `INNER JOIN` table2 `ON` relationship) `INNER JOIN` table3 `ON` relationship);
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#### **LEFT (OUTER) JOIN**: returns all records from the left table (table1), and the matched records from the right table (table2)
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* `SELECT` column_names `FROM` table1 `LEFT JOIN` table2 `ON` table1.column_name=table2.column_name;
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### **RIGHT (OUTER) JOIN**: returns all records from the right table (table2), and the matched records from the left table (table1)
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* `SELECT` column_names `FROM` table1 `RIGHT JOIN` table2 `ON` table1.column_name=table2.column_name;
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#### **FULL (OUTER) JOIN**: returns all records when there is a match in either left or right table
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* `SELECT` column_names `FROM` table1 ``FULL OUTER JOIN`` table2 `ON` table1.column_name=table2.column_name;
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#### **Self JOIN**: a regular join, but the table is joined with itself
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* `SELECT` column_names `FROM` table1 T1, table1 T2 `WHERE` condition;
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### View Queries
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#### **CREATE**: create a view
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* `CREATE VIEW` view_name `AS SELECT` column1, column2 `FROM` table_name `WHERE` condition;
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#### **SELECT**: retrieve a view
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* `SELECT` * `FROM` view_name;
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#### **DROP**: drop a view
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* `DROP VIEW` view_name;
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### Altering Table Queries
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#### **ADD**: add a column
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* `ALTER TABLE` table_name `ADD` column_name column_definition;
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#### **MODIFY**: change data type of column
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* `ALTER TABLE` table_name `MODIFY` column_name column_type;
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#### **DROP**: delete a column
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* `ALTER TABLE` table_name `DROP COLUMN` column_name;
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### Creating Table Query
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### **CREATE**: create a table
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* `CREATE TABLE` table_name `(` <br />
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`column1` `datatype`, <br />
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`column2` `datatype`, <br />
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`column3` `datatype`, <br />
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`column4` `datatype`, <br />
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`);`
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