This course teaches you how to design a relational database and how to write SQL. It covers all the important SQL statements, including CREATE, INSERT, UPDATE, DELETE, SELECT, ALTER, and DROP, and provide some insight into entity-relationship model design. This course is based on my on-campus teaching at colleges and comes with a mid-term project and a final project. There is no prerequisite for this course.
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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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When working in the SQL Server, we may have to check some other databases other than the current one which we are working. In that scenario we may not be sure that does we have access to those Databases?. In this article we discuss the list of databases that are available for the current logged user in SQL Server
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In SSMS, we many of may noticed System Databases under the Database Folder. But how many of us knows its purpose?. In this article lets discuss about the System Databases in SQL Server.
Fig. 1 System Databases
There are five system databases, these databases are created while installing SQL Server.
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Define: SQL [pron. “sequel”] – stands for Structured Query Language (SQL), used by databases to model and manage tabular/relational datasets; a set of standardized Data Definition Language (DDL) functions to create tables, views, and define relational schema models, and Data Manipulation Language (DML) to query, insert, and modify data in the tables.
*Read-only select queries are technically part of its Data Query Language (DQL) group. Still, operationally, many refer to it as DML because it can do more than read-only queries.
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This article will introduce the concept of SQL recursive. Recursive CTE is a really cool. We will see that it can often simplify our code, and avoid a cascade of SQL queries!
The recursive queries are used to query hierarchical data. It avoids a cascade of SQL queries, you can only do one query to retrieve the hierarchical data.
First, what is a CTE? A CTE (Common Table Expression) is a temporary named result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. For example, you can use CTE when, in a query, you will use the same subquery more than once.
A recursive CTE is one having a subquery that refers to its own name!
Recursive CTE is defined in the SQL standard.
A recursive CTE has this structure:
In this example, we use hierarchical data. Each row can have zero or one parent. And it parent can also have a parent etc.
Create table test (id integer, parent_id integer); insert into test (id, parent_id) values (1, null); insert into test (id, parent_id) values (11, 1); insert into test (id, parent_id) values (111, 11); insert into test (id, parent_id) values (112, 11); insert into test (id, parent_id) values (12, 1); insert into test (id, parent_id) values (121, 12);
For example, the row with id 111 has as ancestors: 11 and 1.
Before knowing the recursive CTE, I was doing several queries to get all the ancestors of a row.
For example, to retrieve all the ancestors of the row with id 111.
While (has parent) Select id, parent_id from test where id = X
With recursive CTE, we can retrieve all ancestors of a row with only one SQL query :)
WITH RECURSIVE cte_test AS ( SELECT id, parent_id FROM test WHERE id = 111 UNION SELECT test.id, test.parent_id FROM test JOIN cte_test ON cte_test.id = test.parent_id
) SELECT * FROM cte_test
It indicates we will make recursive
It is the initial query.
It is the recursive expression! We make a jointure with the current CTE!
Replay this example here
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