Using dependent packages to directly implement paging and SQL statements

        <dependency>
            <artifactId>mybatis-plus-boot-starter</artifactId>
            <groupId>com.baomidou</groupId>
            <version>3.3.1</version>
        </dependency>

  There are many methods in this package , This is mainly about LambdaQueryWrapper class .

For example, you need to student Select name It's not empty 、 also age Greater than 18 Of the students ：

The entity classes are as follows ：

@Data
@TableName("student")
public class Student{

    @TableId(value = "id", type = IdType.AUTO)
    private Long id;

    private String name;

    private Integer age;

    private String address;
}

  application LambdaQueryWrapper class ：

LambdaQueryWrapper<Student> queryWrapper = new LambdaQueryWrapper();

queryWrapper.isNotNull(Student::name);//  The name is not empty 

queryWrapper.gt(Student::age, 18);//  Older than 18 year 

  Then combine this filter with paging

        <dependency>
            <artifactId>pagehelper-spring-boot-starter</artifactId>
            <groupId>com.github.pagehelper</groupId>
            <version>1.3.0</version>
        </dependency>

pageHelper Provide PageHelper and PageInfo The method in

int pageNum = 0;

int pageSize = 10;

PageHelper.startPage(pageNum, pageSize);//  Open paging 

——pageNum: The current number of pages ,pageSize: Page size

PageInfo<Student> pageInfo = new PageInfo<>(studentMapper.selectList(queryWrapper));

Combine these two parts ：

@Service
@AllArgsConstructor
public class StudentServiceImpl implements StudentService{

private final StudentMapper studentMapper;

@Override
public List<Student> selectByPage() {

    int pageNum = 0;

    int pageSize = 10;

    PageHelper.startPage(pageNum, pageSize);//  Open paging 

    LambdaQueryWrapper<Student> queryWrapper = new LambdaQueryWrapper();

    queryWrapper.isNotNull(Student::name);//  The name is not empty 

    queryWrapper.gt(Student::age, 18);//  Older than 18 year 

    PageInfo<Student> pageInfo = new PageInfo<>(studentMapper.selectList(queryWrapper));
    
    return pageInfo.getList();

}

}

and studentMapper When using , We need to pay attention to , In the corresponding Mapper Interface should inherit BaseMapper< Entity class >.

Here, for example. ：

public interface StudentMapper extends BaseMapper<Student> {

}

meanwhile , The way to inject here is @AllArgsConstructor And final combination , Rather than using @Resource, Pay attention here , Use @AllArgsConstructor And final when ,@Value It will fail , Get no value , If you want to use @Value, It can be used @RequiredArgsConstructor, But use @RequiredArgsConstructor Can only be used to @NotNull or final The value of takes effect .

LambdaQueryWrapper Common methods ：

Function name	explain	Example	SQL paraphrase
eq	be equal to	eq("name"," Program ")	name=' Program '
ne	It's not equal to	ne("name"," Program ")	name<>' Program '
gt	Greater than	gt("age",18)	age>18
ge	Greater than or equal to	ge("age",18)	age>=18
lt	Less than	lt("age",18)	age<18
le	Less than or equal to	le("age",18)	age<=18
between	stay … Between	between("age",18,30)	age between 18 and 30
notBetween	be not in … Between	notBetween("age",18,30)	age not between 18 and 30
like	% value %	like("name"," hat ")	name like '% hat %'
notLike	not % value %	notLike("name"," hat ")	name not like '% hat %'
likeLeft	% value	likeLeft("name"," hat ")	name like '% hat '
likeRight	value %	likeRight("name"," hat ")	name like ' hat %'
isNull	Value is empty	isNull("name")	name is null
isNotNull	The value is not empty.	isNotNull("name")	name is not null
in	stay … In	in("age","18","20",30")	age in (18, 20, 30)
notIn	be not in … In	notIn("age","18","20",30")	age not in (18, 20, 30)
inSql	in sql sentence	inSql("age","select age from stu where id=1")	age in (select age from stu where id=1)
notInSql	not in sql sentence	notInSql("age","select age from stu where id=1")	age not in (select age from stu where id=1)
groupBy	from … grouping	groupBy("name", "age")	group by name, age
orderBy	according to … Sort	orderBy(true, false, "name", "age")	order by name desc, age desc notes ： take false Change it to true, Then the sorting field is in ascending order
orderByAsc	according to … Ascending order	orderByAsc( "name", "age")	order by name asc, age asc
orderByDesc	according to … In descending order	orderByDesc( "name", "age")	order by name desc, age desc
having	Screening	having("sum(age) > {0} and order = {1}", 10, "01")	having sum(age) > 10 and order = "01"
or	perhaps	eq("name", " Program ").or().gt("age", 18)	name = " Program " or age > 18 notes ： Don't use or perhaps and According to shimmer and
and	also	and(e -> e.eq("name", " Program ").or().gt("age", 18))	and(name = " Program " or age > 18)
apply	Splicing	apply("sum(age) > {0}", 10)	sum(age) > 10
last	Ignore the optimization rules and splice directly to the end	last("where id = 12")	sql The statement is added directly to the end notes ： Yes SQL Injection risk
exists	Specify a subquery , Detect the existence of lines	exists("select name from stu where age > 49")	exists(select name from stu where age > 49) notes ： Return to true or false, Not a specific value
notExists	Specify a subquery , Detection line does not exist	notExists("select name from stu where age > 49")	not exists(select name from stu where age > 49)
nested	Normal nesting , No and perhaps or	(e -> e.eq("name", " Program ").or().gt("age", 18))	(name = " Program " or age > 18)