SQL SELECT Statement

Basic SELECT Statement

SELECT*| {[DISTINCT] column|expression [alias],...}

fromtable;

. Select identifies which columns are selected.

. From identifies the table from which to select.

1. Select selects all columns or select specific columns;

All columns: SELECT * from departments;

Specific columns: SELECT department_id, location_id from departments;

Attention:

• SQL language is not case sensitive.

• SQL can be written in one line or multiple lines

• Keywords cannot be abbreviated or branch

• The clauses are usually written in branches.

• Use indentation to improve the readability of the statement.

2. Binding operator (+-*/)

Operator Precedence (*/+-), parentheses, the operation in parentheses is performed first.

Example: SELECT last_name, Salary, 12* (salary+100) from employees;

3. Using aliases

Alias for column:

• Renames a column.

• Easy to calculate.

• Immediately following the column name, you can also add the keyword ' as ' between the column name and the alias, using double quotation marks to include spaces or special characters in the alias and case sensitivity.

Example 1:

SELECT last_name as name, commission_pct comm from employees;

Example 2:

Select Last_Name "Name", salary*12 "annual salary" from employees;

4. Connector | |

Connector:

• Concatenate columns and columns with characters.

• Use ' | | ' Said.

• can be used to ' synthesize ' columns.

As: System.out.println (123 + "Hello" + 123);//123hello123

Example: SELECTlast_name| | job_id as "Employees" from Employees;

Example: 650) this.width=650; "src=" Http://s3.51cto.com/wyfs02/M01/6F/C2/wKioL1WngP-ToahvAADpgXuMbtA057.jpg "title=" Picture 1.png "alt=" Wkiol1wngp-toahvaadpgxumbta057.jpg "/>

5. String

• The string can be a character, a number, a date in the SELECT list.

• Dates and characters can only appear in single quotation marks.

• Each time a row is returned, the string is output once

Example: SELECT last_name| | ' is a ' | | job_id

As "Employee Details"

From employees;

Example: 650) this.width=650; "src=" Http://s3.51cto.com/wyfs02/M02/6F/C4/wKiom1Wnf9byEHU2AAEAO6ujfvM047.jpg "title=" Picture 2.png "alt=" Wkiom1wnf9byehu2aaeao6ujfvm047.jpg "/>

6. Delete duplicate rows

Delete duplicate rows using the keyword ' DISTINCT ' in the SELECT clause.

Example:

SELECT DISTINCT department_id

From employees;

This article is from the "Test Road" blog, be sure to keep this source http://yangliqin.blog.51cto.com/8836122/1675405

SQL SELECT Statement