Gain insights into SQL by exploring database creation, data manipulation, and advanced queries. Learn about practical applications and practice for SQL interviews to build confidence and readiness.

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The ability to work SQL is becoming an increasingly in-demand skill, both for software developers and people in less technical roles. If you’re interested in learning SQL and have no prior experience with it, then this course will be your light in a dark tunnel.

You’ll start by covering the basics of SQL such as how to create a database, how to insert, query, and update data. You’ll also learn fundamental concepts that developers and data scientists use everyday such as multi-table operations, nested queries, and how to set up views. 

Throughout, you’ll get to execute SQL queries in your browser and see results in real-time - you won’t need to worry about set-up.

At the end of this course, you’ll also get some hands-on practice with common SQL interview questions, so when the time comes, you’ll be ready and confident to answer any question that comes your way. Let’s get started!

An Introductory Guide to SQL

# SIGNAL and RESIGNAL

__SIGNAL__ statement is a way to return an error to a handler or a client application. The sender can choose which error characteristics (like the  error number or message text) to return. __SIGNAL__ provides a way to handle unexpected events that may lead to application termination. By raising errors, we can provide information to the error handler for a graceful exit rather than an abrupt termination.

The __SIGNAL__ keyword is used to raise errors. It is followed by a __SQLSTATE value__ or a named condition defined with a SQLSTATE value. __SIGNAL__ cannot be associated with a MySQL error code. The SQLSTATE value 45000 is generic and can be used to catch any un-handled user defined exceptions. 


The __SET__ clause is used to return information about the error. The  diagnostic information about an error includes __MYSQL_ERRNO, MESSAGE_TEXT, CLASS_ORIGIN, SUBCLASS_ORIGIN, CONSTRAINT_CATALOG, CONSTRAINT_SCHEMA, CONSTRAINT_NAME, CATALOG_NAME, SCHEMA_NAME, TABLE_NAME, COLUMN_NAME__, and __CURSOR_NAME__. The sender can choose to send more than one of these items as a comma separated list.


## Syntax
> SIGNAL **SQLSTATE | condition_name**
>
> SET **condition_information_item1** = **val1**,
    **condition_information_item2** = **val2**, ... ;



The __RESIGNAL__ statement is similar to the SIGNAL statement. It is used to raise warnings and errors. __RESIGNAL__ passes on the information about an error condition to a handler. The __SQLSTATE value__ and the __SET__ clause are optional and __RESIGNAL__ can be used alone in which case it just passes the error without modifying the error information. When the __SET__ clause is used with __RESIGNAL__, it is used to modify the error attributes like changing the error number or the message text.
__RESIGNAL__ can only be used within the scope of a condition handler while the __SIGNAL__ statement can be used anywhere in a stored procedure.


## Syntax
> RESIGNAL **[SQLSTATE | condition_name]**
>
> [SET **condition_information_item1** = **val1**,
    **condition_information_item2** = **val2**, ...];


>Connect to the terminal below by clicking in the widget. Once connected, the command line prompt will show up. Enter or copy and paste the command **./DataJek/Lessons/58lesson.sh** and wait for the MySQL prompt to start-up.

In this lesson we will learn how to raise errors and warnings using SIGNAL and RESIGNAL statements.

Introduction

Basic SQL

Joins

Nested Queries

Multi Table Operations

Views

Stored Procedures

Triggers

Miscellaneous

Practice & Prep

Epilogue

SIGNAL and RESIGNAL

SIGNAL and RESIGNAL

Syntax