動態SQL:根據不同的條件生成不同的不同的sql語句

1.if使用

<select id="selectByIdName" resultType="User">
    select * from user where 1=1
    <if test="id!=null and id!=''">
        and id=#{id}
    </if>
    <if test="username!=null and username!=''">
        and username=#{username}
    </if>
</select>
 

//介面：
public interface UserMapper {
    User selectByIdName(@Param("id")int id,@Param("username")String username);
}
//呼叫:
    UserMapper userMapper = session.getMapper(UserMapper.class);
    User user =userMapper.selectByIdName(3,"lisi");
    System.out.println(user);

2.where使用

簡介:<where></where> 元素只會在至少有一個子元素的條件返回 SQL 子句的情況下才去插入“WHERE”子句。而且，若語句的開頭為“AND”或“OR”，where 元素也會將它們去除。
例如:select * from user where and username=#{username} and password=#{password}將會變成
select * from user where username=#{username} and password=#{password}
 

<select id="selectByNamePwd1" resultType="User">
    select * from user
    <where>
        <if test="username!=null and username!=''">
            username=#{username}
        </if>
        <if test="password!=null and password!=''">
            and password=#{password}
        </if>
    </where>
</select>
 

//介面：
public interface UserMapper {
        User selectByNamePwd1(@Param("username") String username,@Param("password")String password);
}
//呼叫:
    UserMapper userMapper = session.getMapper(UserMapper.class);
    User user=userMapper.selectByNamePwd1("lisi","123");
    System.out.println(user);

3.choose, when, otherwise使用

簡介:有時我們不想應用到所有的條件語句，而只想從中擇其一項。針對這種情況，MyBatis 提供了 choose 元素，它像 Java 中的 if-else if - else 語句。
注意，如果<choose></choose>下的<when>都滿足條件，將會只輸出前者(即choose只會選擇一個生成語句)

<select id="selectByNamePwd1" resultType="User">
    select * from user
    <where>
        <choose>
            <when test="username!=null and username!=''">
            	username=#{username}
            </when>
            <when test="password!=null and password!=''">
            	password=#{password}
            </when>
        </choose>
    </where>
</select>

//介面：
public interface UserMapper {
        User selectByNamePwd1(@Param("username") String username,@Param("password")String password);
}
//呼叫:
    UserMapper userMapper = session.getMapper(UserMapper.class);
    User user=userMapper.selectByNamePwd1("1","1");
    System.out.println(user);

4.set使用

簡介:去掉最後一個","

<update id="upd" parameterType="User">
    update user
    <set>
        此處加id=#{id},是為了防止下面兩個if判斷都不成功導致語法錯誤(update user where id=#{id})
        id=#{id},
        <if test="username!=null">
            username=#{username},
        </if>
        <if test="password!=null">
            password=#{password},
        </if>
    </set>
    where id=#{id}
</update>

//介面：
    public interface UserMapper {
            int upd(User user);
    }
//呼叫:
    User user=new User(3,"updname","updpwd");
    int index = userMapper.upd(user);
    System.out.println(index);
    session.commit();

5.trim使用

簡介:where、set原型

prefix:字首增加
prefixOverrides:字首刪除
suffix:字尾增加
suffixOverrides:字尾刪除
注意:執行順序，先刪除內容再增加內容
where功能:
<select id="selectByNamePwd2" resultType="User">
    select * from user
    <trim prefix="where" prefixOverrides="and">
        <if test="username!=null and username!=''">
            username=#{username}
        </if>
        <if test="password!=null and password!=''">
            and password=#{password}
        </if>
    </trim>
</select>

//介面：
    public interface UserMapper {
        User selectByNamePwd2(@Param("username") String username,@Param("password")String password);
    }
//呼叫:
    User user=userMapper.selectByNamePwd2("","111");
    System.out.println(user);

6.bind使用

簡介:給引數重新賦值，用於模糊查詢、在原內容前或者後新增內容

模糊查詢
<select id="selectById1" resultType="User">
    <bind name="username" value="'%'+username+'%'"/>
    select * from user where username like #{username}
</select>

//介面：
    public interface UserMapper {
        User selectById1(@Param("id")int id,@Param("username") String username);
    }
//呼叫:
		User user=userMapper.selectById1(2,"li");
        System.out.println(user);

7.foreach使用

簡介:迴圈遍歷引數內容，還具備在內容的前後新增內容，還具備新增分隔符功能
適用於in查詢中 select * from user where id in ( , , , )
批量新增(注意:mybatis中foreach效率比較低)      
insert int user values (id1,username1,password1),(id2,username2,password2),(id3,username3,password3)

collection:要遍歷的集合
item:迭代變數
open:迴圈後左側新增的內容
close:迴圈後右側新增的內容
separator:每次迴圈時,元素之間的分隔符
<select id="selectByIdList" resultType="User">
    select * from user where id in
    <foreach collection="idList" item="idList" open="(" close=")" separator=",">
        #{idList}
    </foreach>
</select>

//介面：
    public interface UserMapper {
        List<User> selectByIdList(@Param("idList")List<Integer> idList);
    }
//呼叫:
    List<Integer> list=new ArrayList<>();
    list.add(2);
    list.add(3);
    List<User> userList=userMapper.selectByIdList(list);
    for(User user:userList){
        System.out.println(user);
    }

8.sql使用

簡介:某些SQL片段如果希望複用，可以使用<sql>定義這個片段
之後在<select>等標籤中使用<include>引用

<select id="selectAll2" resultType="User">
    select <include refid="mysql"></include>
    from user
</select>

<sql id="mysql">
    id,username
</sql>

//介面：
    public interface UserMapper {
         List<User> selectAll2();
    }
//呼叫:
    List<User> userList=userMapper.selectAll2();
    for(User user : userList){
        System.out.println(user);
    }