In computer programming, a callback is a reference to executable code, or a piece of executable code, that is passed as an argument to other code. This allows a lower-level software layer to call a subroutine (or function) defined in a higher-level layer.
You receive a package in the mail. While this package may possibly include some data, the most important piece of this package is that it includes an envelope addressed back to the package's sender (postage paid for of course!) You may use some data in the package, or generate some of your own, but you for sure have a way of recontacting (calling back!..) your original sender.
- understanding callbacks
- what are callbacks
- jquery callbacks
- asynchronous javascript
- callback hell
- callbacks explaied
- callback tutorial
- passing parameters to callbacks
- Alternative ways to pass a callback to a function in Javascript
- passing arguments to callback
- callback functions explained
- Callback function
- Declaring and Using Callbacks
- callbacks in components
- Using promises
- what are callback functions
- what is callback
- callback example
- avoiding callback hell
- callbacks for beginners
- call back tutorial video
- interact with a database using callbacks
- master callbacks
- video tutorial
- callbacks usig real life analogy
- callbacks in plain english
Take a look at the text representation of a database. It looks similar to an Excel spreadsheet, which is a helpful way of visualizing how databases store information.
| id | first_name | last_name | gender | |
|---|---|---|---|---|
| 1 | Farley | Kitten | male | farley@gmail.com |
| 2 | Tiger | Chaw | male | tiger@gmail.com |
| 3 | Snickers | Meowas-alot | male | snickers@yahoo.com |
| 4 | Kittu | Doggie | male | Kittu@gmail.com |
| 5 | Sparky | Doggie | female | sparkey@gmail.com |
Each column (or in DB speak field) has a name. We can describe the structure of this table independently of what data is in it. We'd write the structure like this:
+------------+ | students | +------------+ | id | | first_name | | last_name | | gender | | email | +------------+
We can use any field names and table names we want, but there are some conventions that we follow for our own sanity. These are conventions — you might see other conventions used elsewhere.
- Table names are pluralized, e.g., "students," "products," "todos," "favorite_foods," "programming_languages," "people," etc.
- We use snake_case_like_this to name fields, rather than camelCaseLikeThis —
- Fields that involve dates or times end in
_ator_on(created_at,updated_at,completed_at, etc.) unless it's really obvious they already refer to something time-related. We'd saybirthdayrather thanborn_on, for example. - When in doubt, make your field and table names as self-explanatory as possible. Avoid field names like
type,kind,status, etc. They could refer to anything! Err on the side of clarity, even if it seems verbose. Other coders will appreciate it, and computers don't care if your field name is 20 characters long rather than 5 characters long.
Each row in a database table should have a primary key. This is a field (or collection of fields) which uniquely identify that row from all other rows. Rails defaults to using a synthetic primary key, which is just an arbitrary, auto-incrementing integer usually denoted by the field name id. It's called "synthetic" because it doesn't have any inherent meaning.
The assumption that the primary key is an auto-incrementing integer called id baked deeply into Relational Databases.
SQLite is a software library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine. SQLite is the most widely deployed SQL database engine in the world. The source code for SQLite is in the public domain.
PostgreSQL is a powerful, open source object-relational database system. It has more than 15 years of active development and a proven architecture that has earned it a strong reputation for reliability, data integrity, and correctness. It runs on all major operating systems, including Linux, UNIX (AIX, BSD, HP-UX, SGI IRIX, Mac OS X, Solaris, Tru64), and Windows. It is fully ACID compliant, has full support for foreign keys, joins, views, triggers, and stored procedures (in multiple languages). It includes most SQL:2008 data types, including INTEGER, NUMERIC, BOOLEAN, CHAR, VARCHAR, DATE, INTERVAL, and TIMESTAMP. It also supports storage of binary large objects, including pictures, sounds, or video. It has native programming interfaces for C/C++, Java, .Net, Perl, Python, Ruby, Tcl, ODBC, among others, and exceptional documentation.
An enterprise class database, PostgreSQL boasts sophisticated features such as Multi-Version Concurrency Control (MVCC), point in time recovery, tablespaces, asynchronous replication, nested transactions (savepoints), online/hot backups, a sophisticated query planner/optimizer, and write ahead logging for fault tolerance. It supports international character sets, multibyte character encodings, Unicode, and it is locale-aware for sorting, case-sensitivity, and formatting. It is highly scalable both in the sheer quantity of data it can manage and in the number of concurrent users it can accommodate. There are active PostgreSQL systems in production environments that manage in excess of 4 terabytes of data.
PostgreSQL has won praise from its users and industry recognition, including the Linux New Media Award for Best Database System and five time winner of the The Linux Journal Editors' Choice Award for best DBMS.
C.R.U.D
create: INSERT
retrieve / read: SELECT
update: UPDATE
delete / destroy: DELETE
--------------------------------------
-- CREATE the TABLE attendees
--------------------------------------
CREATE TABLE attendees (
id INTEGER(11) AUTO_INCREMENT NOT NULL,
-- Makes a string column called "first_name" which cannot contain null --
first_name VARCHAR(30) NOT NULL,
-- Makes a string column called "first_name" which cannot contain null --
last_name VARCHAR(30) NOT NULL,
-- Makes a string column called "first_name" which cannot contain null --
gender VARCHAR(6) NOT NULL CHECK (Gender IN ('male', 'female', 'other'))
email VARCHAR(30),
-- Makes a sting column called "pet_name" --
PRIMARY KEY (id)
);
--------------------------------------
-- asking questions using INSERT
--------------------------------------
-- can i add an attendee to the attendees table?
INSERT INTO attendees
(first_name, last_name, gender, email)
VALUES
('jane', 'smith', 'female','abc@hi.com')
-- can i add an attendee with no email?
INSERT INTO attendees
(first_name, last_name, gender)
VALUES
('jane', 'smith', 'female')
-- can i change the order of the columns in my insert statement?
INSERT INTO attendees
(gender, first_name, last_name, email)
VALUES
('female', 'jane', 'smith', 'abc@biz.com')
--------------------------------------
-- asking questions using SELECT
--------------------------------------
-- can i get a list of the women in this table?
SELECT *
FROM attendees
WHERE gender = 'female'
-- can i get a list of the men ...?
SELECT *
FROM attendees
WHERE gender = 'male'
-- can i get a list of the other ...?
SELECT *
FROM attendees
WHERE gender = 'other'
-- can i get a list of attendees whose last name starts with the letter 'S'?
SELECT *
FROM attendees
WHERE last_name LIKE 'S%'
-- can i get a sorted list of attendees with emails ending in .biz?
SELECT *
FROM attendees
WHERE email LIKE '%biz'
ORDER BY last_name, first_name
--------------------------------------
-- changing data using UPDATE
--------------------------------------
-- can i change the email address of an attendee if i know their id?
UPDATE attendees
SET email = '123@abc.com'
WHERE id = 1
-- can i change the email address of an attendee without their id?
UPDATE attendees
SET email = '123@abc.com'
WHERE email = 'abc@biz.com'
-- can i change the email address of several attendees at once?
UPDATE attendees
SET email = '123@abc.com'
WHERE last_name LIKE 'S%'
--------------------------------------
-- deleting data using DELETE
--------------------------------------
-- can i delete an attendee record if i know their id?
DELETE from attendees
WHERE id = 1
-- can i delete an attendee record without their id?
DELETE from attendees
WHERE email = '123@abc.com'
-- can i delete everything from the table?
DELETE from attendees
WHERE 1 = 1
SQL Server: ALTER TABLE Statement This SQL Server tutorial explains how to use the ALTER TABLE statement in SQL Server (Transact-SQL) to add a column, modify a column, drop a column, rename a column or rename a table with syntax and examples.
https://www.techonthenet.com/sql_server/tables/alter_table.php
ALTER TABLE table_name
ADD column_name column-definition;Example
Let's look at an example that shows how to add a column in an SQL Server table using the ALTER TABLE statement.
For example:
ALTER TABLE attendees
ADD has_pet BOOLEAN NOT NULL;ALTER TABLE attendees
ADD pet_age INTEGER(10);ALTER TABLE attendees
ADD pet_name VARCHAR(30);Add multiple columns in table
You can use the ALTER TABLE statement in SQL Server to add multiple columns to a table.
ALTER TABLE table_name
ADD column_1 column-definition,
column_2 column-definition,
...
column_n column_definition;For example:
ALTER TABLE attendees
ADD city VARCHAR(50),
state VARCHAR(40);
Modify column in table
ALTER TABLE table_name
ALTER COLUMN column_name column_type;
ALTER TABLE attendees
ALTER COLUMN last_name VARCHAR(75) NOT NULL;
Drop column in table
You can use the ALTER TABLE statement in SQL Server to drop a column in a table.
ALTER TABLE table_name
DROP COLUMN column_name;
Example
ALTER TABLE attendees
DROP COLUMN last_name;Rename column in table You can not use the ALTER TABLE statement in SQL Server to rename a column in a table. However, you can use sp_rename, though Microsoft recommends that you drop and recreate the table so that scripts and stored procedures are not broken.
sp_rename 'table_name.old_column_name', 'new_column_name', 'COLUMN';
Example
sp_rename 'attendees.last_name', 'lname', 'COLUMN';
Rename table
sp_rename 'old_table_name', 'new_table_name';Example
sp_rename 'attendees', 'students';