SQL joins — A refresher

Hello everyone. I’m writing this post as a part of my journey with MySQL and since joins is a confusing thing in the SQL, I’m explaining this by simplest terms as possible. I’m taking data from a schema and posting the code here. We will be going through each and every joins in MySQL and let’s hop in. First, let’s see what joins is all about and then types of joins and their explanations with examples.

Note : I have outsourced some data and values from Sakila dataset and also some definitions from various websites for easy understanding.

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MySQL join :

  • MySQL join is a function that is join two or more tables.
  • As we handle many kinds of data in future, we are gonna link many tables
  • These tables are linked by columns which are common between tables
  • In case of not common columns, a different kind of join is used
  • Sometimes, same table is joined or precisely compared to get values

Types of MySQL join :

Imagine Venn diagrams in your mind so that this will be very easy to grasp. A little bit of tenth standard and 11th standard will surely help. Let’s refresh our knowledge in Venn a bit. Venn diagrams are fun and in our boards they help us to get more marks. In this, we will look forward using those here.

Venn functions — [union, intersection, set difference]

  • Inner Join — Works like intersection
  • Right Join — Set difference (B-A)
  • Left Join — Set difference (A-B)
  • Outer Join — Union
  • Cross join : This sorta works like nested for loop (ref python)

Inner join : Inner join is basically a join that includes the common columns in both the tables. This works like a sort of intersection in our mathematics. In intersection, the A intersection B, the result takes the common value between A and B. Similarly, in inner join or common join (ms access there is only 3 types of join) — the join happens between common columns. Let’s jump into the syntax :

Select table1.col1, table1.col2, table2.col1, table2.col2 from table1 JOIN table2 on common col of table1 = common col of table2;

Note : we can use the groupby and having and orderby and where clauses in this syntax.

Breaking down the syntax :

_Select _— the common select syntax

table1.col1, table1.col2, table2.col1, table2.col2— here table1,table2 denotes where the column is taken from

Join_ : Here the join denotes the join function indicating that a join has happened here_

_on _— On statement is used to denote on which column the table is joined

**Example **: Use sakila schema in the MySQL

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Highlighted portion shows where the join has happened

We are gonna find the full name and the address of the customers by joining customer and address table as c and ad respectively.

Result :

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We can see full_name and full address of the person

We can also do multiple inner joins on various tables to get the desired output.

**Using multiple tables : **Use the Sakila schema

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Here we have joined the customer, address, city and country

We are gonna get the full_name, address and the country_name from the tables customer, address, city, country

**Result **:

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We get the country name in the table

Thus by this way we can also join multiple tables and we can include various statements and execute our block. Make sure that we join on the correct columns — to avoid error.

#analytics-vidhya #data-science #dbms #database #mysql

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SQL joins — A refresher
Cayla  Erdman

Cayla Erdman

1594369800

Introduction to Structured Query Language SQL pdf

SQL stands for Structured Query Language. SQL is a scripting language expected to store, control, and inquiry information put away in social databases. The main manifestation of SQL showed up in 1974, when a gathering in IBM built up the principal model of a social database. The primary business social database was discharged by Relational Software later turning out to be Oracle.

Models for SQL exist. In any case, the SQL that can be utilized on every last one of the major RDBMS today is in various flavors. This is because of two reasons:

1. The SQL order standard is genuinely intricate, and it isn’t handy to actualize the whole standard.

2. Every database seller needs an approach to separate its item from others.

Right now, contrasts are noted where fitting.

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Karlee  Will

Karlee Will

1621561800

Your Ultimate Guide to SQL Join: CROSS JOIN

CROSS JOIN is in the spotlight. This article finishes our small series of SQL JOIN-related publications.

SQL Server CROSS JOIN is the simplest of all joins. It implements a combination of 2 tables without a join condition. If you have 5 rows in one table and 3 rows in another, you get 15 combinations. Another definition is a Cartesian Product.

Now, why would you want to combine tables without a join condition? Hang on a bit because we are getting there. First, let’s refer to the syntax.

#sql server #cross join #inner join #outer join #sql join #sql

Karlee  Will

Karlee Will

1621522680

Your Ultimate Guide to SQL Join: INNER JOIN

Inner join, outer join, cross join? What gives?

It’s a valid question. I once saw a Visual Basic code with T-SQL codes embedded in it. The VB code retrieves table records with multiple SELECT statements, one SELECT * per table. Then, it combines multiple result sets into a record set. Absurd?

To the young developers who did it, it was not. But when they asked me to evaluate why the system was slow, that issue was the first to catch my attention. That’s right. They never heard of SQL joins. In fairness to them, they were honest and open to suggestions.

How do you describe SQL joins? Perhaps, you remember one song – Imagine by John Lennon:

You may say I’m a dreamer, but I’m not the only one.

I hope someday you’ll join us, and the world will be as one.

#sql server #inner join #sql join #t-sql #sql

Karlee  Will

Karlee Will

1621554240

Your Ultimate Guide to SQL Joins: OUTER JOIN

Outer join is at the center stage today. And this is part 2 of your ultimate guide to SQL joins. If you missed part 1, here’s the link.

By the looks of it, outer is the opposite of inner. However, if you consider the outer join this way, you’ll be confused. To top that, you don’t have to include the word outer in your syntax explicitly. It’s optional!

But before we dive in, let’s discuss nulls concerning outer joins.

Nulls and OUTER JOIN

When you join 2 tables, one of the values from either table can be null. For INNER JOINs, records with nulls won’t match, and they will be discarded and won’t appear in the result set. If you want to get the records that don’t match, your only option is OUTER JOIN.

Going back to antonyms, isn’t that the opposite of INNER JOINs? Not entirely, as you will see in the next section.

#sql server #inner join #outer join #sql join #sql

Cayla  Erdman

Cayla Erdman

1596441660

Welcome Back the T-SQL Debugger with SQL Complete – SQL Debugger

When you develop large chunks of T-SQL code with the help of the SQL Server Management Studio tool, it is essential to test the “Live” behavior of your code by making sure that each small piece of code works fine and being able to allocate any error message that may cause a failure within that code.

The easiest way to perform that would be to use the T-SQL debugger feature, which used to be built-in over the SQL Server Management Studio tool. But since the T-SQL debugger feature was removed completely from SQL Server Management Studio 18 and later editions, we need a replacement for that feature. This is because we cannot keep using the old versions of SSMS just to support the T-SQL Debugger feature without “enjoying” the new features and bug fixes that are released in the new SSMS versions.

If you plan to wait for SSMS to bring back the T-SQL Debugger feature, vote in the Put Debugger back into SSMS 18 to ask Microsoft to reintroduce it.

As for me, I searched for an alternative tool for a T-SQL Debugger SSMS built-in feature and found that Devart company rolled out a new T-SQL Debugger feature to version 6.4 of SQL – Complete tool. SQL Complete is an add-in for Visual Studio and SSMS that offers scripts autocompletion capabilities, which help develop and debug your SQL database project.

The SQL Debugger feature of SQL Complete allows you to check the execution of your scripts, procedures, functions, and triggers step by step by adding breakpoints to the lines where you plan to start, suspend, evaluate, step through, and then to continue the execution of your script.

You can download SQL Complete from the dbForge Download page and install it on your machine using a straight-forward installation wizard. The wizard will ask you to specify the installation path for the SQL Complete tool and the versions of SSMS and Visual Studio that you plan to install the SQL Complete on, as an add-in, from the versions that are installed on your machine, as shown below:

Once SQL Complete is fully installed on your machine, the dbForge SQL Complete installation wizard will notify you of whether the installation was completed successfully or the wizard faced any specific issue that you can troubleshoot and fix easily. If there are no issues, the wizard will provide you with an option to open the SSMS tool and start using the SQL Complete tool, as displayed below:

When you open SSMS, you will see a new “Debug” tools menu, under which you can navigate the SQL Debugger feature options. Besides, you will see a list of icons that will be used to control the debug mode of the T-SQL query at the leftmost side of the SSMS tool. If you cannot see the list, you can go to View -> Toolbars -> Debugger to make these icons visible.

During the debugging session, the SQL Debugger icons will be as follows:

The functionality of these icons within the SQL Debugger can be summarized as:

  • Adding Breakpoints to control the execution pause of the T-SQL script at a specific statement allows you to check the debugging information of the T-SQL statements such as the values for the parameters and the variables.
  • Step Into is “navigate” through the script statements one by one, allowing you to check how each statement behaves.
  • Step Over is “execute” a specific stored procedure if you are sure that it contains no error.
  • Step Out is “return” from the stored procedure, function, or trigger to the main debugging window.
  • Continue executing the script until reaching the next breakpoint.
  • Stop Debugging is “terminate” the debugging session.
  • Restart “stop and start” the current debugging session.

#sql server #sql #sql debugger #sql server #sql server stored procedure #ssms #t-sql queries