Query of conditional statements

Conditional statements

• Where clause to filter the data table, the data symbols condition appears in the result set
• The syntax is as follows:

select 字段1,字段2... from 表名 where 条件;
例：
select * from students where id=1;

• Where the back support multiple operators, conditional processing
  • Comparison operation
  • logic operation
  • Fuzzy query
  • Range queries
  • Empty judgment

Comparison Operators

• Equal to: =
• Greater than:>
• Greater than or equal:> =
• Less than: <
• Less than or equal: <=
• Does not mean:! = Or <>

Example 1: Query Missy age

select age from students where name='小乔'

Example 2: Query students below 20 years of age

select * from students where age<20

Example 3: The query is not in Beijing students home

select * from students where hometown!='北京'

Exercise:

1、查询学号是'007'的学生的身份证号
2、查询'1班'以外的学生信息
3、查询年龄大于20的学生的姓名和性别

Logical Operators

• and
• or
• not

Example 1: Query female students younger than 20

select * from students where age<20 and sex='女'

Example 2: Query or female students 'class 1' students

select * from students where sex='女' or class='1班'

Example 3: non-student inquiry in Tianjin

select * from students where not hometown='天津'

Exercise:

1、查询河南或河北的学生
2、查询'1班'的'上海'的学生
3、查询非20岁的学生

Fuzzy query

• like
• % Represents any number of any character
• _ Represents an arbitrary character

Example 1: Query student surnamed Sun

select * from students where name like '孙%'

Example 2: Query surnamed Sun and the name is a word of student

select * from students where name like '孙_'

Example 3: Query student named Joe

select * from students where name like '%乔'

Example 4: Query name containing white students

select * from students where name like '%白%'

Exercise:

1、查询姓名为两个字的学生
2、查询姓百且年龄大于20的学生
3、查询学号以1结尾的学生

Range queries

• It represents a non-contiguous in the range of

Example 1: Query hometown students in Beijing or Shanghai or Guangdong

select * from students where hometown in('北京','上海','广东')

• between ... and ... represents a continuous range

Example 2: Query for students ages 18-20 in

select * from students where age between 18 and 20

Exercise:

1、查询年龄在18或19或22的女生
2、查询年龄在20到25以外的学生

Empty judgment

• Note: null and '' are different
• Empty sentence is null

Example 1: The query does not fill out the identity of the student

select * from students where card is null

• Sentenced to a non-null is not null

Example 2: Query filled out student ID card

select * from students where card is not null