我们在写复杂的SQL语句时，稍不注意就可能出现错误。
而动态SQL，通过 if, choose, when, otherwise, trim, where, set, foreach标签，可组合成非常灵活的SQL语句。
既能提高开发人员的效率，又可以减少出错。
1.foreach标签
1.1foreach实现in集合
- java代码
public void testForEach() {
List<String> idList = new ArrayList<>();
idList.add("1");
idList.add("2");
idList.add("3");
List<User> users = userMapper.selectUserByUserIdList(idList);
users.forEach(System.out::println);
}
- SQL：
<select id="selectUserByUserIdList" resultType="cn.no7player.model.User">
SELECT
id, name, age, password
FROM
user
WHERE id IN
<foreach collection="list" open="(" close=")" separator="," item="id">
#{id}
</foreach>
</select>
说明：如果传入的是List集合，则<foreach>中的 collection默认是"list"
<foreach>标签中collection属性
如果在selectUserByUserIdList( ) 传参时用了 @Param("xxx") 注解，则collection为"xxx"
java代码：
List<User> selectUserByUserIdList(@Param("idLists") List<String> idList);
SQL语句：
<select id="selectUserByUserIdList" resultType="cn.no7player.model.User">
SELECT
id, name, age, password
FROM
user
WHERE id IN
<foreach collection="idLists" open="(" close=")" separator="," item="id" index="i">
#{id}
</foreach>
</select>
1.2foreach实现批量插入
java代码：
public void testForEach2() {
List<User> userList = new ArrayList<>();
userList.add(new User(0,"GEM", 28, "999999"));
userList.add(new User(0,"JAY", 36, "777777"));
int i = userMapper.insertUsers(userList);
System.out.println(i);
}
SQL语句：
<insert id="insertUsers" useGeneratedKeys="true" keyProperty="id">
INSERT INTO
user(name, age, password)
VALUES
<foreach collection="list" separator="," item="user">
(
#{user.name},
#{user.age},
#{user.pwd}
)
</foreach>
</insert>
