What are the SQL query statements? 04/26 Update SLTechnology News&Howtos

What are the SQL query statements?

2025-04-26 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > Development >

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This article introduces the relevant knowledge of "what are the SQL query sentences?". In the operation of actual cases, many people will encounter such a dilemma. Then let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

SQL stands for structured query language and is a language for managing data in a database. SQL consists of commands and declarative statements that act as instructions to the database so that it can perform tasks. You can use the SQL command to create tables in a database, add and change large amounts of data, search for data to quickly find specific content, or delete tables together.

The basic structure of database

Before you begin, you should understand the hierarchical structure of the database.

An SQL database is a collection of related information stored in a table. Each table has columns that describe the data in it, and rows that contain the actual data. A field is a single data in a row. Therefore, to get the data we need, we need to be specific.

For example, a remote company can have multiple databases. To see a complete list of their databases, we can type SHOW DATABASES; and partition on the Employees database.

The output will look like this:

+-+ | Databases | +-+ | mysql | | information_schema | | employees | | test | | sys | +-+

A database can have multiple tables. Taking the above example as an example, to view the different tables in the employees database, we can do SHOW TABLES in employees;. The forms can be Engineering, Product, Marketing, and Sales for different teams owned by the company.

Then, all tables are made up of different columns that describe the data.

To view different columns, use Describe Engineering;. For example, a bill of materials can have a defined single attribute sample column employee_id,first_name,last_name,email,country, and salary.

This is the output:

A table also consists of rows, which are individual entries in the table. For example, a guild includes employee_id,first_name,last_name,email,salary, and country according to entries. These lines will define and provide information about a person on the engineering team.

Basic SQL query

Everything you can do with the data follows the CRUD acronym.

CRUD represents the four main actions we perform when querying the database: create, read, update, and delete.

We CREATE provide information in the database, we READ/ retrieve this information from the database, we UPDATE/ manipulate it, and we can DELETE if we want.

Below we will introduce some basic SQL queries and their syntax to get started.

SQLCREATE DATABASE statement

To create a database named engineering, we can use the following code:

CREATE DATABASE engineering;SQLCREATE TABLE statement CREATE TABLE table_name (column1 datatype, column2 datatype, column3 datatype)

This query creates a new table in the database.

It provides a name for the table, and we want the different columns that our table has to be passed in.

We can use a variety of data types. Some of the most common are: INT,DECIMAL,DATETIME,VARCHAR,NVARCHAR,FLOAT, and BIT.

From the above example, this might be similar to the following code:

CREATE TABLE engineering (employee_id int (6) NOT NULL,first_name varchar (20) NOT NULL,last_name varchar (25) NOT NULL,email varchar (255) NOT NULL,country varchar (30), salary decimal (10mem2) NOT NULL)

The table we created from this data looks similar to the following:

EMPLOYEE_IDFIRST_NAMELAST_NAMEEMAILCOUNTRYSALARY coach

SQLALTER TABLE statement

After we create the table, we can modify it by adding another column to it.

ALTER TABLE table_name ADD column_name datatype

For example, if we want, we can birthday add a column to the existing table by typing:

ALTER TABLE engineeringADD birthday date

Now our table will look like this:

EMPLOYEE_IDFIRST_NAMELAST_NAMEEMAILCOUNTRYSALARYBIRTHDAY coach

SQLINSERT statement

This is how we insert data into the table and create new rows. This is part of the CCRUD.

INSERT INTO table_name (column1, column2, column3,..) VALUES (value1, 'value2', value3,..)

In the INSERT INTO section, we can specify the columns to populate with information.

VALUES is the information we want to store in it. This creates a new record in the table, which is a new row.

Whenever we insert string values, they are enclosed in single quotation marks,'.

For example:

INSERT INTO table_name (employee_id,first_name,last_name,email,country,salary) VALUES (1) (2) (2) (2) (2) (2) (2) (2)

It should look like this:

EMPLOYEE_IDFIRST_NAMELAST_NAMEEMAILCOUNTRYSALARY1TimmyJonestimmy@gmail.comUSA2500.002KellySmithksmith@gmail.comUK1300.00SQLSELECT statement

This statement gets the data from the database. It is part of RCRUD.

SELECT column1,column2FROM table_name

From our previous example, this would look like this:

SELECT first_name,last_nameFROM engineering

Output:

The SELECT statement points to the specific column from which we want to get the data, which we want to display in the result.

The FROM section determines the table itself.

This is another example of SELECT:

SELECT * FROM table_name

The asterisk * will get all the information from the table we specify.

SQLWHERE statement

WHERE enables us to understand our query in more detail.

If we want to filter our Engineering table to search for employees with a specific salary, we will use WHERE.

SELECT employee_id,first_name,last_name,email,countryFROM engineeringWHERE salary > 1500

The table in the previous example:

EMPLOYEE_IDFIRST_NAMELAST_NAMEEMAILCOUNTRYSALARY1TimmyJonestimmy@gmail.comUSA2500.002KellySmithksmith@gmail.comUK1300.00

You will now have the following output:

The filter passes and states the result of meeting the condition-that is, it indicates that only the person whose salary is on the line more than 1500.

SQL AND,OR and BETWEEN service provider

These operators allow you to make the query more specific by adding more conditions to the WHERE statement.

The AND operation occurs on two conditions, both of which must be rows that true wants to display in the result.

SELECT column_nameFROM table_nameWHERE column1 = value1 AND column2 = value2

The OR operator accepts two conditions, one of which must be true to display the row in the result.

SELECT column_nameFROM table_nameWHERE column_name = value1 OR column_name = value2

The BETWEEN operator filters out numbers or text within a specific range.

SELECT column1,column2FROM table_nameWHERE column_name BETWEEN value1 AND value2

We can also combine these operators with each other.

Suppose we behave like this:

EMPLOYEE_IDFIRST_NAMELAST_NAMEEMAILCOUNTRYSALARY1TimmyJonestimmy@gmail.comUSA2500.002KellySmithksmith@gmail.comUK1300.003JimWhitejwhite@gmail.comUK1200.764Jos é Luis Martinezjmart @ gmail.comMexico1275.875EmiliaFischeremfis@gmail.comGermany2365.906DelphineLavignelavigned@gmail.comFrance2108.007LouisMeyerlmey@gmail.comGermany2145.70

If we use the following statement:

SELECT * FROM engineeringWHERE employee_id BETWEEN 3 AND 7 AND country = 'Germany'

We will get this output:

This will select all comlumns 3 and 7 employee_id between German country AND.

SQLORDER BY statement

ORDER BY sorts by the columns we mentioned in the SELECT statement.

It sorts the results and displays them in descending or ascending order of letters or numbers (the default is ascending order).

We can specify the following command: ORDER BY column_name DESC | ASC.

SELECT employee_id, first_name, last_name,salaryFROM engineeringORDER BY salary DESC

In the above example, we sort the salaries of the employees on the engineering team and sort them in descending order of numbers.

SQLGROUP BY statement

GROUP BY lets us combine rows with the same data and similarities.

It is helpful to arrange duplicate data with items that appear multiple times in the table.

SELECT column_name, COUNT (*) FROM table_nameGROUP BY column_name

Here COUNT (*) calculates each row individually and returns the number of rows in the specified table while retaining duplicate rows.

SQLLIMIT statement

LIMIT allows you to specify the maximum number of rows that should be returned in the result.

This is useful when dealing with large datasets that may cause queries to take a long time to run. It can save you time by limiting the results you get.

SELECT column1,column2FROM table_nameLIMIT number;SQLUPDATE statement

This is how we update a row in the table. This U is part of CRUD.

UPDATE table_name SET column1 = value1, column2 = value2 WHERE condition

The WHERE condition specifies the record to edit.

UPDATE engineeringSET country = 'Spain'WHERE employee_id = 1

Our previous watch:

Now it looks like this:

EMPLOYEE_IDFIRST_NAMELAST_NAMEEMAILCOUNTRYSALARY1TimmyJonestimmy@gmail.comSpain2500.002KellySmithksmith@gmail.comUK1300.003JimWhitejwhite@gmail.comUK1200.764Jos é Luis Martinezjmart @ gmail.comMexico1275.875EmiliaFischeremfis@gmail.comGermany2365.906DelphineLavignelavigned@gmail.comFrance2108.007LouisMeyerlmey@gmail.comGermany2145.70

This updates the country of residence column for employees with an ID of 1.

We can also update the information in the table JOIN with values from another table.

UPDATE table_nameSET table_name1.column_name1 = table_name2.column_name1 table_name1.column_name2 = table_name2.column2FROM table_name1JOIN table_name2 ON table_name1.column_name = table_2.column_name;SQLDELETE statement

DELETE is part of DCRUD. This is how we delete records from the table.

The basic syntax is as follows:

DELETE FROM table_name WHERE condition

For example, in our engineering example, it might look like this:

DELETE FROM engineeringWHERE employee_id = 2

This deletes the record of the employee with id 2 on the engineering team.

SQLDROP COLUMN statement

To delete a specific column from the table, we will do the following:

ALTER TABLE table_name DROP COLUMN column_name;SQLDROP TABLE statement

To delete the entire table, we can do this:

This is the end of DROP TABLE table_name; 's "what are the SQL query statements"? thank you for your reading. If you want to know more about the industry, you can follow the website, the editor will output more high-quality practical articles for you!

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