Sql basics

reference: http://www.tutorialspoint.com/sql

important: constrains:

SQL constraints are used to specify rules for the data in a table.
If there is any violation between the constraint and the data action, the action is aborted by the constraint.

In SQL, we have the following constraints:

NOT NULL - Indicates that a column cannot store NULL value
UNIQUE - Ensures that each rows for a column must have a unique value
PRIMARY KEY - A combination of a NOT NULL and UNIQUE. Ensures that a column (or combination of two or more columns) have an unique identity which helps to find a particular record in a table more easily and quickly
FOREIGN KEY - Ensure the referential integrity of the data in one table to match values in another table
CHECK - Ensures that the value in a column meets a specific condition
DEFAULT - Specifies a default value when specified none for this column

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Select statement:

Slecet From Where Order by [DESC/ASC] group by COUNT/SUM

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Join Keyword:

what is spurious tuple?

A spurious tuple is, basically, a record in a database that gets created when two tables are joined badly. In database-ese, spurious tuples are created when two tables are joined on attributes that are neither primary keys nor foreign keys.

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CROSS JOIN (avoid to use):

The SQL CROSS JOIN produces a result set which is the number of rows in the first table multiplied by the number of rows in the second table, if no WHERE clause is used along with CROSS JOIN. This kind of result is called as Cartesian
Product.

Select *
FROM table1
CROSS JOIN table2;

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Full OUTER JOIN:
The FULL OUTER JOIN keyword returns all rows from the left table (table1) and from the right table (table2).The FULL OUTER JOIN keyword combines the result of