An important component of any relational database system is the relationship between tables. This relationship facilitates connecting two tables based on data that they ‘have in common’. This connection is accomplished using a JOIN, which is an operation that matches rows from one table to the rows in another. The matching is done in such a manner that the columns from both the tables placed side by side although they may have come from separate tables. It must be said though, the fewer tables being joined is the faster the query will operate. This article gives an introduction to joins and their uses in the relational database engine.
Let’s create tables: CUSTOMERS and ORDERS as follows:
CUSTOMERS ( cust_id [PK] , cust_fname, cust_lname, phone, address, country);
ORDERS ( order_id [PK] , order_date, reqd_date, ship_date, cust_id [FK] );
Scenario: We need to see a list of customers and details of their respective orders.
This is the simplest type of join where each row in one table is matched with all other rows in another table. Only if the join condition evaluates to true a row is. The data in common, must be true for both tables involved in the JOIN.
SELECT customers.cust_fname, orders.order_id, orders.order_date FROM customers INNER JOIN orders ON customer.cust_id = orders.cust_id;
Interestingly enough (doesn’t work for all engines), you can also write an INNER JOIN query with a WHERE condition.
SELECT customers.cust_fname, orders.order_id, orders.order_date FROM customers, orders WHERE customers.cust_id = orders.cust_id;
This JOIN retrieves all records in the table that is on the (literally) LEFT side of the join condition AND…any columns that match from the table on the RIGHT side of the condition. So a so customer with no orders will be returned from the customer table, but NULL values are returned for the columns in the orders table for the rows that do not match the JOIN condition.
SELECT customers.cust_fname, orders.order_id, orders.order_date FROM customers LEFT JOIN orders ON customer.cust_id = orders.cust_id;
This JOIN does the inverse of the LEFT JOIN. It returns all records from the table on the right, with any rows that might match the condition from the left table. In the same way, NULL values are returned for the columns in the left table for the rows that do not match the JOIN condition.
SELECT customers.cust_fname, orders.order_id, orders.order_date FROM customers RIGHT JOIN orders ON customer.cust_id = orders.cust_id;
A cross join returns as many combinations of all the rows contained in the joined tables. That is,each row of the left table concatenated with every row in the right table. Therefore, if there are total M rows in Table A and there are N rows in Table B, a cross join will produce M x N rows. Two ways to accomplish this CROSS JOIN are as follows:
SELECT customers.cust_fname, orders.order_id, orders.order_date FROM customers CROSS JOIN orders;
SELECT customers.cust_fname, orders.order_id, orders.order_date FROM customers, orders;
Typically, JOINs are used to establish a master-detail kind of data representation and are essential for pulling meaningful data from multiple related tables. Multiple tables can be joined in a singular select statement, but remember that the query will slow down for each table and more so, the number of records being returned.
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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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
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
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.
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
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:
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