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MyBatis Plus的条件构造器
1.组装查询条件
组装查询其实很简单，可以支持条件的链式编程：
查询用户名包含a，年龄在 10 - 20 之间并且邮箱不为空的用户：
@Test
void contextLoads() {
// 查询用户名包含a，年龄在 10 - 20 之间并且邮箱不为空的用户
QueryWrapper<User> wrapper = new QueryWrapper<>();
wrapper.like("name","a")
.between("age",10,20)
.isNotNull("email");
List<User> users = userMapper.selectList(wrapper);
users.forEach(System.out::println);
}==> Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 AND (name LIKE ? AND age BETWEEN ? AND ? AND email IS NOT NULL) ==> Parameters: %a%(String), 10(Integer), 20(Integer) <== Columns: id, name, age, email, is_delete <== Row: 2, dada, 11, 111@qq.com, 0 <== Row: 4, dahe, 12, 34567@qq.com, 0 <== Total: 2
2.组装排序条件
查询用户信息，按照年龄的降序排序，若年龄相同，则按照id升序排序：
@Test
void contextLoads() {
// 查询用户信息，按照年龄的降序排序，若年龄相同，则按照id升序排序
QueryWrapper<User> wrapper = new QueryWrapper<>();
wrapper.orderByDesc("age").orderByAsc("id");
List<User> users = userMapper.selectList(wrapper);
users.forEach(System.out::println);
}==> Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 ORDER BY age DESC,id ASC ==> Parameters: <== Columns: id, name, age, email, is_delete <== Row: 3, dahe, 33, 34567@qq.com, 0 <== Row: 4, dahe, 12, 34567@qq.com, 0 <== Row: 2, dada, 11, 111@qq.com, 0 <== Total: 3
3.使用Lambda表达式解决条件优先级
将用户名中包含有a并且（年龄大于20或邮箱为null）的用户信息查询出来：
Lambda中的条件优先执行！
@Test
void contextLoads() {
// 将用户名中包含有a并且（年龄大于20或邮箱为null）的用户信息查询出来
QueryWrapper<User> wrapper = new QueryWrapper<>();
wrapper.like("name", "a")
.and(i -> i.gt("age", 20).or().isNull("email"));
List<User> users = userMapper.selectList(wrapper);
users.forEach(System.out::println);
}==> Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 AND (name LIKE ? AND (age > ? OR email IS NULL)) ==> Parameters: %a%(String), 20(Integer) <== Columns: id, name, age, email, is_delete <== Row: 3, dahe, 33, 34567@qq.com, 0 <== Total: 1
4.组装select语句
有时我们不想查询数据库某张表的所有字段，可以使用maps集合选择只查询某些字段的值：
@Test
void contextLoads() {
QueryWrapper<User> wrapper = new QueryWrapper<>();
wrapper.select("name","email");
List<Map<String, Object>> maps = userMapper.selectMaps(wrapper);
maps.forEach(System.out::println);
}==> Preparing: SELECT name,email FROM user WHERE is_delete=0
==> Parameters:
<== Columns: name, email
<== Row: dada, 111@qq.com
<== Row: dahe, 34567@qq.com
<== Row: dahe, 34567@qq.com
<== Total: 3
Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@237add]
{name=dada, email=111@qq.com}
{name=dahe, email=34567@qq.com}
{name=dahe, email=34567@qq.com}5.组装子查询
@Test
void contextLoads() {
QueryWrapper<User> wrapper = new QueryWrapper<>();
wrapper.inSql("id","select id from user where id > 1");
List<User> users = userMapper.selectList(wrapper);
users.forEach(System.out::println);
}==> Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 AND (id IN (select id from user where id > 1)) ==> Parameters: <== Columns: id, name, age, email, is_delete <== Row: 2, dada, 11, 111@qq.com, 0 <== Row: 3, dahe, 33, 34567@qq.com, 0 <== Row: 4, dahe, 12, 34567@qq.com, 0 <== Total: 3
6.使用UpdateWrapper实现修改功能
将名字中包含a的记录修改：
@Test
void contextLoads() {
// 将名字中包含a的记录修改
UpdateWrapper<User> wrapper = new UpdateWrapper<>();
wrapper.like("name","a");
wrapper.set("name","hello").set("age",18);
userMapper.update(null, wrapper);
}JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@36c07c75] will not be managed by Spring ==> Preparing: UPDATE user SET name=?,age=? WHERE is_delete=0 AND (name LIKE ?) ==> Parameters: hello(String), 18(Integer), %a%(String) <== Updates: 3
7.模拟开发中组装条件的情况
真实的开发中往往有这样的情况：我们不知道前端传来的条件是什么，此时无法正常的拼接sql，那么就需要在后端进行相应的判断：
@Test
void contextLoads() {
String name = null;
Integer ageBegin = 18;
Integer ageEnd = 28;
QueryWrapper<User> wrapper = new QueryWrapper<>();
if (StringUtils.isNotBlank(name)) {
wrapper.like("name", name);
}
if (ageBegin != null) {
wrapper.ge("age", ageBegin);
}
if (ageEnd != null) {
wrapper.le("age", ageEnd);
}
List<User> users = userMapper.selectList(wrapper);
users.forEach(System.out::println);
}==> Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 AND (age >= ? AND age <= ?) ==> Parameters: 18(Integer), 28(Integer) <== Columns: id, name, age, email, is_delete <== Row: 3, hello, 18, 34567@qq.com, 0 <== Row: 4, hello, 18, 34567@qq.com, 0 <== Total: 2
但是，这样的判断我们认为过于的简陋和复杂，事实上，MyBatis Plus的条件构造器中为我们封装了解决此类问题的一些重载方法：
@Test
void contextLoads() {
String name = null;
Integer ageBegin = 18;
Integer ageEnd = 28;
QueryWrapper<User> wrapper = new QueryWrapper<>();
wrapper.like(StringUtils.isNotBlank(name), "name", name);
wrapper.ge(ageBegin != null, "age", ageBegin);
wrapper.le(ageEnd != null, "age", ageEnd);
List<User> users = userMapper.selectList(wrapper);
users.forEach(System.out::println);
}==> Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 AND (age >= ? AND age <= ?) ==> Parameters: 18(Integer), 28(Integer) <== Columns: id, name, age, email, is_delete <== Row: 3, hello, 18, 34567@qq.com, 0 <== Row: 4, hello, 18, 34567@qq.com, 0 <== Total: 2
8.LambdaQueryWrapper和LambdaUpdateWrapper
使用LambdaQueryWrapper和LambdaUpdateWrapper可以防止我们的字段名修改导致的程序错误或字段名写错
例如：
@Test
void contextLoads() {
String name = null;
Integer ageBegin = 18;
Integer ageEnd = 28;
LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
wrapper.like(StringUtils.isNotBlank(name), User::getName, name);
wrapper.ge(ageBegin != null, User::getAge, ageBegin);
wrapper.le(ageEnd != null, User::getAge, ageEnd);
List<User> users = userMapper.selectList(wrapper);
users.forEach(System.out::println);
}==> Preparing: SELECT id,name,age,email,is_delete FROM user WHERE is_delete=0 AND (age >= ? AND age <= ?) ==> Parameters: 18(Integer), 28(Integer) <== Columns: id, name, age, email, is_delete <== Row: 3, hello, 18, 34567@qq.com, 0 <== Row: 4, hello, 18, 34567@qq.com, 0 <== Total: 2
LambdaUpdateWrapper的使用方法类似
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