SQL triggers

Syntax:

CREATE TRIGGER trigger_name trigger_time trigger_event
ON table_name
FOR EACH ROW
BEGIN
...
END

Dropping trigger:

DROP TRIGGER table_name.trigger_name

Example 1:

CREATE TABLE account (acct_num INT, amount DECIMAL(10,2));

CREATE TRIGGER ins_sum BEFORE INSERT ON account FOR EACH ROW SET @sum = @sum + NEW.amount;

SET @sum = 0;
mysql> INSERT INTO account VALUES(137,14.98),(141,1937.50),(97,-100.00);
mysql> SELECT @sum AS 'Total amount inserted';

Example 2:

CREATE TRIGGER `tutorial`.`before_delete_carts`
    BEFORE DELETE ON `trigger_carts` FOR EACH ROW
    BEGIN
        DELETE FROM trigger_cart_items WHERE OLD.cart_id = cart_id;
    END

Example 3:

CREATE TRIGGER `after_update_cost`
    AFTER UPDATE ON `trigger_items_cost` FOR EACH ROW
    BEGIN
       UPDATE trigger_items
       SET price = (NEW.cost * 1.3)
       WHERE item_id = NEW.item_id;
    END

SQL tests

Example data 1:
==============================
|FirstName      |LastName    |
==============================
|'Peter'        |'Hansen'    |
|'Peter'        |'Jackson'   |
|'John'         |'Nilsen'    |
|'David'        |'Jackson'   |
==============================

Question 1: Lets have Example data 1;
How much rows will return this query:

SELECT * FROM Persons A, Persons B WHERE
(A.FirstName = B.FirstName Or  A.LastName = B.LastName) AND NOT
(A.FirstName = B.FirstName AND A.LastName = B.LastName)

SQL GROUP BY and HAVING in examples

Here we show you the usage HAVING and GROUP BY. As an advance, we have an example with GROUP BY multiple columns.
In first section you have got the data schema. Later you have some tasks on it. After that you have the solutions for the tasks.

SELECT into SELECT

Lets explain the syntax with an example. The input data is:

having syntax in SQL

sql HAVING example:
SELECT Customer,SUM(OrderPrice) FROM Orders
GROUP BY Customer
HAVING SUM(OrderPrice)<2000


sql HAVING Syntax:
SELECT column_name, aggregate_function(column_name)
FROM table_name
WHERE column_name operator value
GROUP BY column_name
HAVING aggregate_function(column_name) operator value


Here is a nice example with having.

Database Normalization

Database Normalization is the process of efficiently organizing data in a database. There are two goals of the normalization process:
* eliminating redundant data (for example, storing the same data in more than one table)
* ensuring data dependencies make sense (only storing related data in a table).

Both of these are worthy goals as they reduce the amount of space a database consumes and ensure that data is logically stored.

The Normal Forms

The database community has developed a series of guidelines for ensuring that databases are normalized. These are referred to as normal forms and are numbered from one (the lowest form of normalization, referred to as first normal form or 1NF) through five (fifth normal form or 5NF). In practical applications, you'll often see 1NF, 2NF, and 3NF along with the occasional 4NF.

First Normal Form (1NF)

First normal form (1NF) sets the very basic rules for an organized database:
* Eliminate duplicative columns from the same table.
* Create separate tables for each group of related data and identify each row with a unique column or set of columns (the primary key).

Second Normal Form (2NF)

Second normal form (2NF) further addresses the concept of removing duplicative data:
* Meet all the requirements of the first normal form.
* Remove subsets of data that apply to multiple rows of a table and place them in separate tables.
* Create relationships between these new tables and their predecessors through the use of foreign keys.

Third Normal Form (3NF)

Third normal form (3NF) goes one large step further:
* Meet all the requirements of the second normal form.
* Remove columns that are not dependent upon the primary key.

Fourth Normal Form (4NF)

Finally, fourth normal form (4NF) has one additional requirement:
* Meet all the requirements of the third normal form.
* A relation is in 4NF if it has no multi-valued dependencies.

Remember, these normalization guidelines are cumulative. For a database to be in 2NF, it must first fulfill all the criteria of a 1NF database.

If you'd like to ensure your database is normalized, explore our other articles in this series:

* Database Normalization Basics
* Putting your Database in First Normal Form
* Putting your Database in Second Normal Form
* Putting your Database in Third Normal Form