DQL where query

-- where Conditional query
-- basic where Statement query
-- Operation symbol
-- <,>,<=,>=,=
-- Parallel relationship &&,and
-- between Conditions 1 and Conditions 2
-- Or the relationship between || ,or
-- in（ aggregate ）
-- is null: A field is empty Or not empty is not null
SELECT * FROM student;

-- demand ： Inquire about student Table middle school students are older than 20 A man of years old
SELECT 
  NAME,
  age 
FROM
  student 
WHERE age > 20 ;

-- demand ： Inquire about student The age in the table is less than or equal to 20 year
SELECT 
  NAME,
  age 
FROM
  student 
WHERE age <= 20 ;

-- Format 2：
SELECT 
  NAME,
  age 
FROM
  student 
WHERE age < 20 
  OR age = 20 ;-- ||(Java Logical symbols in ), Recommended or
  
SELECT * FROM student; -- Display full table data

-- demand ： The age of inquiry is 20~30 People between the ages of （ Include both ends ）
-- Format 1：
SELECT 
  NAME,
  age 
FROM
  student 
WHERE age >= 20 && age <= 30 ;

-- Format 2：
SELECT 
  NAME,
  age 
FROM
  student 
WHERE age >= 20 
  AND age <= 30 ;
  
-- between Conditions 1 and Conditions 2; Query between that range
SELECT 
  NAME,
  age 
FROM
  student 
WHERE age BETWEEN 20 
  AND 30 ;

-- demand ： The age in the student table is not 20 A man of years old
SELECT 
  NAME,
  age 
FROM
  student 
WHERE age != 20 ; -- !=(java Middle operator )

SELECT 
  NAME,
  age 
FROM
  student 
WHERE age <> 20 ;-- mysql:<>: It's not equal to

SELECT * FROM student;

-- need ： The English score is null Student information （name）
SELECT 
  NAME 
FROM
  student 
WHERE english IS NULL ;-- A field is null: Field name is null

-- The English score is not null Student information
SELECT 
  NAME 
FROM
  student 
WHERE english IS NOT NULL ;

-- demand : Age is 18,28,30 The name of the year old student
SELECT 
  NAME 
FROM
  student 
WHERE age = 18 
  OR age = 28 
  OR age = 30 ;
-- Optimize the above format in( Aggregate data )
SELECT 
  `name`,
  math,
  english 
FROM
  student 
WHERE age IN (18, 28, 30) ;