Database SQL language 01 where condition

DQL Language

DQL( Data Query Language Data query language )

• Query database data , Such as SELECT sentence
• Simple single table query or complex query and nested query of multiple tables are the core of database language , The most important statement
• The most frequently used statement

SELECT grammar

SELECT [ALL | DISTINCT]
{
   * | table.* | [table.field1[as alias1][,table.field2[as alias2]][,...]]} 
FROM table_name [as table_alias]
[left | right | inner join table_name2]	--  The joint query 
[WHERE ...]	--  Specify the conditions to be met for the result 
[GROUP BY ...]	--  Specifies which fields the results are grouped by 
[HAVING]	--  Filtering the grouped records must meet the secondary conditions 
[ORDER BY ...]	--  Specifies that query records are sorted by one or more criteria 
[LIMIT {
   [offset,]row_count | row_count OFFSET offset}];
--  Specify which records to query from 
 Be careful  : [ ]  Parentheses represent optional  , { } The brackets mean you have to choose 

--  Query all data column results in the table  ,  use  **" \* "**  Symbol ;
	
--  Check all student information  
SELECT * FROM student;	
	
--  Query specified column ( Student number  ,  full name ) 
SELECT studentno,studentname FROM	student;

AS Clause as alias

effect ：

• You can take a new alias for the data column
• A new alias can be given to the table
• The calculated or summarized results can be replaced by another new name

--  Here is the alias for the column ( Of course as Key words can be omitted ) 
SELECT studentno AS  Student number ,studentname AS  full name 	FROM	student;	
			
--  Use as You can also alias a table  
SELECT studentno AS  Student number ,studentname AS  full name 	FROM	student AS	s;
			
--  Use as, Give the query a new name  
-- CONCAT() Function concatenation string  
SELECT CONCAT(' full name :',studentname) AS  New name 	FROM	student;	

DISTINCT Use of keywords

effect : Get rid of SELECT Duplicate records in the returned record result of query ( Returns the same value for all columns ) , Just return one

-- #  Check which students took the exam ( Student number )  Remove duplicates  
SELECT * FROM result; --  Check exam results  
SELECT studentno FROM result; --  Check which students took the exam  
SELECT DISTINCT studentno FROM result; --  understand :DISTINCT  Remove duplicates  , ( The default is ALL)

Columns that use expressions

Expressions in the database : It's usually a text value , The column value , NULL , Functions and operators, etc
Application scenarios :

• SELECT Statement return result column
• SELECT Statement ORDER BY , HAVING Use in sub sentences
• DML Statement where Using expressions in conditional statements

-- selcet Expressions can be used in queries  
SELECT @@auto_increment_increment; --  Query self increasing step size  
SELECT VERSION(); --  Query version No  
SELECT 100*3-1 AS  The result of the calculation is ; --  expression  
	
--  Students' test scores will be raised by one point  
SELECT studentno,StudentResult+1 AS ' After scoring ' FROM	result;

avoid SQL The return result contains ’ . ’ , ’ * ’ And parentheses interfere with the development of language programs .

where Conditional statements

effect ： Used to retrieve data from a data table eligible The record of
The search criteria may consist of one or more logical expressions , The result is usually true or false .

Logical operators

Example :

--  Queries that meet the criteria (where)
SELECT Studentno,StudentResult FROM result;

--  Check the test results in 95-100 Between 
SELECT Studentno,StudentResult
FROM result
WHERE StudentResult>=95 AND StudentResult<=100;

-- AND Or you could write it as  &&
SELECT Studentno,StudentResult
FROM result
WHERE StudentResult>=95 && StudentResult<=100;

--  Fuzzy query ( The corresponding words : Precise query )
SELECT Studentno,StudentResult
FROM result
WHERE StudentResult BETWEEN 95 AND 100;

--  except 1000 Classmate No , I want the results of other students 
SELECT studentno,studentresult
FROM result
WHERE studentno!=1000;

--  Use NOT
SELECT studentno,studentresult
FROM result
WHERE NOT studentno=1000;

Fuzzy query ： Comparison operator

Be careful ：

• Only records of numeric data type can perform arithmetic operation ;
• Only data of the same data type can be compared ;

test ：

--  Fuzzy query  between and \ like \ in \ null

-- =============================================
-- LIKE
-- =============================================
--  Check the student number and name of the student surnamed Liu 
-- like Wildcard used in combination  : % ( representative 0 To any character ) _ ( A character )
SELECT studentno,studentname FROM student
WHERE studentname LIKE ' Liu %';

--  Inquire about Liu , There's only one word behind it 
SELECT studentno,studentname FROM student
WHERE studentname LIKE ' Liu _';

--  Inquire about Liu , There are only two words behind it 
SELECT studentno,studentname FROM student
WHERE studentname LIKE '  Liu  ';

--  The query name contains   Jia   The word 
SELECT studentno,studentname FROM student
WHERE studentname LIKE '% Jia %';

--  If the name contains special characters, you need to use escape symbols  '\'
--  Custom escape key : ESCAPE ':'

-- =============================================
-- IN
-- =============================================
--  The student ID is 1000,1001,1002 Student name of 
SELECT studentno,studentname FROM student
WHERE studentno IN (1000,1001,1002);

--  Inquiry address in Beijing , nanjing , Students in Luoyang, Henan Province 
SELECT studentno,studentname,address FROM student
WHERE address IN (' Beijing ',' nanjing ',' Luoyang, Henan ');

-- =============================================
-- NULL  empty 
-- =============================================
--  The student's date of birth was not filled in 
--  Don't write directly =NULL ,  It's a mistake  ,  use  is null
SELECT studentname FROM student
WHERE BornDate IS NULL;

--  Query the students who filled in the date of birth 
SELECT studentname FROM student
WHERE BornDate IS NOT NULL;

--  Search for students who don't have a home address ( Empty string not equal to null)
SELECT studentname FROM student
WHERE Address='' OR Address IS NULL;