SQL statement

SQL is a data management tool commonly used in many fields. ( SQL statement ) If you do not have a lot of time to do deep research on computer science and have more data that needs to be analyzed, then use SQL as a simple but very effective SQL.Almost all high-level programming languages have SQL support.MS Visual Basic, MS Access, MS Visual Foxpro, DBase, DB2, Paradox, Oracle ….

SQL statement By Techills

First, SQL is used in database management languages (database) and runs on a single machine.Due to the rapid development of the need to build large client/server model, many software using SQL language was born, which is typical of MS SQL Server, Oracle, Sybase. .

sql statement

SQL statement

In the client/server model, the entire database is centralized on the server, and all data processing is performed on the server by SQL statements.The client is only used to update data for the server or retrieve information from the server.

In the emerging field of the Internet, the SQL language plays a more important role.It is used to quickly create dynamic Web pages.Dynamic web pages usually have content extracted from the database.SQL can be used as a glue between a database and a Web page.When the user requests, SQL performs access to information in the database on the server and displays the results on the Web site.And SQL is also a tool to update information for that database.

( SQL statement By Techills )

What is SQL? What is the importance of SQL?

 

SQL first stands for Structured Query Language, which is roughly translated as structured query language.This means that SQL works only with table data such as Foxpro, DBase, Access, etc. If you only work with text files such as Winword, sound … then you can not apply SQL.

SQL objects are data tables and these tables consist of multiple columns and rows.Columns called fields and rows are records of the table.Columns with names and data types define the structure of the table.When the table is organized systematically for a purpose, some work has a database.

In the daily work, there are many different types of tables such as employee lists, list of departments, payroll, balance sheet … When this table is saved to database, it is possible to use SQL to analyze, Process information in it.The performance of SQL is usually a new table.For example, if you want SQL to be retrieved from the Employees List of Employees who have a house in Hanoi, SQL will return a table containing only employees who have a house in Hanoi.

SQL statement By Techills

SQL is also used to update information for existing databases.For example, you need to delete all delivery notes two years ago, for example.You can open the Inventory tab and search and delete each one manually, both time-consuming and misleading.You should use SQL with the execution time in seconds and ensure there is no confusion or omission at all.In such cases the SQL statement result is not the new table but rather the table that was added to update the data as required.

SQL always works on tables. SQL is used to analyze, synthesize data from existing tables, create new table forms, modify the structure, data of existing tables.After executing one or more sequences of SQL statements, it is possible to have enough metrics to generate reports according to job requirements.Because the goal of database management programs is to manage data stored in tables, SQL is used very often in every job.

Why is SQL so popular?

SQL statement

SQL statement

One of the reasons why SQL is so popular is that SQL is easy to use.You do not have to be a computer expert, you do not need to know programming, but you can still use SQL to handle your work.This is due to the intuitive tools that make SQL statements easy and effective, which are Query queries in software that uses SQL.Each Query is a fully built and logged SQL statement that can be used at any time.The product of a query is a new table, which can be used in other queries.That way we can have a string of Query before we get to the end result.These queries are specially designed to express your questions about stored data and when it runs, it returns the results according to your requirements.

Using Query is easy and less error-prone, even with just a mouse to create a complete SQL statement.The only requirement to use Query is that you have to master the structure of your database.When you design a query, just use your mouse to select the data tables you want to include in the query, and then select the fields to include, then write the query with a name.And you can use it immediately or use it to build other Query.We will build a SQL statement using Query to better understand its use.

SQL statement By Techills

Suppose you had two tables of data as Employees and Invoices in MS Access.The Employee list contains the employees in the company and the invoice records all invoices used when selling.The Employee Code field is used to connect between two tables.The requirement is to report the sales amount of each employee.

We will build a query to make the request.Create a Query that includes the Invoice and Employee table combined to get the total sales amount for each employee.After selecting New Query, the table selection dialog will appear and select the two tables needed by double-clicking on the name of each table or pressing Add after selecting the table.

Select the table and select the required fields to output the results by double-clicking on the field name.The selected fields will appear in the grid below.In this case, double click on Employee Name and Money.Since we need to have total sales amount for each employee, we must markTotalsfrom the view menu and select group byEmployee Name andSumforSumand result as shown.So you have finished designing a query and can see the SQL statement that was automatically generated by choosing SQL from the View menu and naming the query with an arbitrary name.

SQL statement By Techills

Then, when you want to see the result of an existing SQL statement, simply run the generated Query by double-clicking it or clicking on the Open button.The results displayed on the screen are the sum of the latest data in the two source tables at the time the query was run.After mastering SQL you have a powerful tool for building programs.

When learning to use a new language, you do not have to repeat from the beginning how to use SQL in that language.The core part of SQL is the same in every language.You only need to learn how to apply SQL in each specific case of that language and the additional improvements.SQL is capable of meeting most of the requirements for updating and analyzing data from tables.

Let’s take a look at some of the main uses of SQL.

  • Selecting certain columns in the data table: We often do not use all of the table information at the same time.SQL can be used to extract only the required columns.
  • Filter records according to different criteria: such as separating the invoices of a certain customer, or print a list of employees from just a few departments.
  • Sort records according to different criteria: Each report type usually requires sorting records in different columns for easy tracking.There are statistic reports sorted by customer, other reports sorted by items sold, although all information is in the same Sales table.You can perform sorting by one or more columns using SQL.Update, delete records across the board under different conditions: for example, when you need to delete all the bills issued 5 years ago …
  • Combining two or more tables horizontally: In the database, each table stores information about an object and the tables are related to each other through the key fields.Use SQL to perform the merging of these tables together through the key fields as in the example above to obtain the required result table.
  • Join two or more tables vertically: When the data is very large or scattered in many places, you may have to manage multiple tables in the same pattern, such as each table for a quarter, month, or company.When you need to aggregate data for the whole year or the whole corporation, you can use SQL to join tables together.
  • Create new table, change existing table structure: for programming.
  • Perform statistical calculations in groups: sum, average, max, min … This is a function that is frequently used to synthesize information before printing the report as the total count of each face. The number of goods sold to each customer …
  • Connect to server data: When the data warehouse is focused on the server in MS SQL Server or Oracle … we must use the SQL command to directly penetrate the database.
  • Combine Web pages with databases with SQL.There are two types of SQL statements, one that aggregates data into a new table (the new table may only exist in memory or be written to disk), and one that only updates the data, the structure of the tables. had.New tables created from SQL statements can be used anywhere in a program that requires tabular data, especially for reports, lists, grid objects, …

All SQL functions can be performed using other tools of software that use SQL.However, SQL has many advantages compared to other ways such as:

  • The command is simple, easy to remember, easy to use and very short.
  • Very high speed due to being optimized with new technologies.
  • Ability to perform complex job requirements …

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If you have mastered the workflow, you should save time and effort by using SQL.A simpler and simpler SQL statement is less error tolerance and faster performance.In languages such as Visual Foxpro or Access, you are provided with an intuitive tool for generating queries – independent SQL statements.Using the Query Builder you may not need to master the SQL syntax and still create complex queries.Once you have created the statement, SQL is automatically generated and you can copy it to other things.

SQL statement

SQL statement

You may be wondering if SQL is so universal that you do not use SQL alone to process data but still have to use other programming tools.SQL is more than capable, but in some cases it must be used in conjunction with other programming tools to get the most out of it.For example, when you need to work with each record, not across the board, when you are working with a particular record and want to switch to working with a record behind it, or you want to Obtain information about the structure of a table (column name, data type, ability to update does not …).The ability of SQL is very large, so it has a wide range of applications in the management of the database by computer.