1.查询selectOne （3.0.3版）
@Test
public void selectMethod() throws SQLException { // 根据ID获取一个对象的数据 Employee employee =employeeMapper.selectOne(new QueryWrapper<Employee>().eq("id","1")); if (employee!=null) {
logger.info("++一个员工信息+++++"+gson.toJson(employee));
} }
其相当于SQL语句：SELECT id,last_name,email,gender,age FROM tbl_employee WHERE id = ?
2.3版本的写法是直接注入一个对象，对象的set值就是查询的条件,同时，ID要是Long或Integer类型，条件一定要唯一，否则，会报数据重复的异常。
Employee employee = new Employee()；
employee.setId(1); Employee result =employeeMapper.selectOne(employee); if (result!=null) {
logger.info("++一个员工信息+++++"+gson.toJson(result));
}
2、查询selectMaps（3.0.3新出）
@Test
public void selectMethod() throws SQLException { // 6、多个Map查询
Employee employee = employeeMapper.selectById(1); QueryWrapper<Employee> employeeQueryWrapper = new QueryWrapper<>(); List<Map<String, Object>> employeeList =employeeMapper.selectMaps(employeeQueryWrapper); if (!employeeList.isEmpty()) {
logger.info("++一个员工信息+++++"+gson.toJson(employeeList));
} }
写法跟其他的查询没有区别，只是返回的结果，变成List<Map<String,Object>>，条件查询跟上边的一样。
返回样式
++一个员工信息+++++[{"gender":"0","last_name":"Marry","id":1,"email":"1","age":55},{"gender":"0","last_name":"Marry","id":2,"email":"1","age":55},{"gender":"0","last_name":"Marry","id":3,"email":"1","age":55},{"gender":"0","last_name":"Marry","id":4,"email":"1","age":55},{"gender":"0","last_name":"Marry","id":12,"email":"1","age":55},{"gender":"0","last_name":"Marry","id":13,"email":"1","age":55},{"gender":"0","last_name":"Marry","id":14,"email":"1","age":55},{"gender":"0","last_name":"Marry","id":15,"email":"1","age":55},{"gender":"0","last_name":"Marry","id":16,"email":"1","age":55},{"gender":"0","last_name":"Marry","id":17,"email":"1","age":55},{"gender":"0","last_name":"Marry","id":18,"email":"1","age":55},{"gender":"0","last_name":"Marry","id":19,"email":"1","age":55},{"gender":"0","last_name":"Marry","id":20,"email":"1","age":55},{"gender":"0","last_name":"Marry","id":21,"email":"1","age":55}] (TestCRUD.java:159) 本来以为是要装多个map进去查询就写成下边的样子，结果不是那个意思
// 6、多个Map查询
Employee employee = employeeMapper.selectById(1); Map<String,Object> map1 = new HashMap<>(16);
map1.put("email",employee.getEmail());
map1.put("age",employee.getAge()); Employee employee2 = employeeMapper.selectById(2);
Map<String,Object> map2 = new HashMap<>(16);
map2.put("email",employee2.getEmail());
map2.put("age",employee2.getAge()); List<Map<String,Object>> mapList = new ArrayList<>();
mapList.add(map1);
mapList.add(map2); QueryWrapper<List<Map<String, Object>>> maps = new QueryWrapper<List<Map<String, Object>>>();
maps.setEntity((mapList)); List<Map<String, Object>> employeeList =employeeMapper.selectMaps(maps);
3、查询selectObjs
@Test
public void selectMethod() throws SQLException { // 7、selectObjs
Employee employee = employeeMapper.selectById(1); List<Object> employeeList =employeeMapper.selectObjs(new QueryWrapper<Employee>().eq("email",employee.getEmail())); if (!employeeList.isEmpty()) {
logger.info("++一个员工信息+++++"+gson.toJson(employeeList));
} }
用法一样，就是返回的结果是个Object类型。
4、统计结果查询selectCount
@Test
public void selectMethod() throws SQLException { // 8、selectCount
Employee employee = employeeMapper.selectById(1); Integer count = employeeMapper.selectCount(new QueryWrapper<Employee>().eq("email", employee.getEmail())); if (count > 0) {
logger.info("++统计结果：+++++" + count);
} }
5、分页查询selectPage
@Test
public void selectMethod() throws SQLException { // 9、分页查询 Integer count = employeeMapper.selectCount(new QueryWrapper<Employee>().between("age", 0, 100)
.eq("gender", 0)); IPage<Employee> employeeIPage = new Page<Employee>(1, 3,count); IPage<Employee> employeeList = employeeMapper.selectPage(employeeIPage,new QueryWrapper<Employee>()
.between("age", 0, 100)
.eq("gender", 0)); if (!employeeList.getRecords().isEmpty()) {
logger.info("++一个员工信息+++++" + gson.toJson(employeeList));
} }
6、分页查询返回map类型的selectMapsPage
@Test
public void selectMethod() throws SQLException { // 10、分页查询Map结果返回
Integer count = employeeMapper.selectCount(new QueryWrapper<Employee>().between("age", 0, 100)
.eq("gender", 0)); IPage<Employee> employeeIPage = new Page<Employee>(1, 3, count); IPage<Map<String, Object>> employeeList = employeeMapper.selectMapsPage(employeeIPage, new QueryWrapper<Employee>()
.between("age", 0, 100)
.eq("gender", 0)); if (!employeeList.getRecords().isEmpty()) {
logger.info("++一个员工信息+++++" + gson.toJson(employeeList));
} }
System.out.println("返回数据："+employeeIPage.getRecords());
System.out.println("总条数："+page.getTotal());
System.out.println("当前页码："+page.getCurrent());
System.out.println("总页码："+page.getPages());
System.out.println("每页显示条数："+page.getSize());
System.out.println("是否有上一页："+page.hasPrevious());
System.out.println("是否有下一页："+page.hasNext());
System.out.println("返回的数据："+page.getRecords());