动态SQL

动态 SQL 是 MyBatis 的强大特性之一。如果你使用过 JDBC 或其它类似的框架，你应该能理解根据不同条件拼接 SQL 语句有多痛苦，例如拼接时要确保不能忘记添加必要的空格，还要注意去掉列表最后一个列名的逗号。利用动态 SQL，可以彻底摆脱这种痛苦。

使用动态 SQL 并非一件易事，但借助可用于任何 SQL 映射语句中的强大的动态 SQL 语言，MyBatis 显著地提升了这一特性的易用性。

如果你之前用过 JSTL 或任何基于类 XML 语言的文本处理器，你对动态 SQL 元素可能会感觉似曾相识。在 MyBatis 之前的版本中，需要花时间了解大量的元素。借助功能强大的基于 OGNL 的表达式，MyBatis 3 替换了之前的大部分元素，大大精简了元素种类，现在要学习的元素种类比原来的一半还要少。

• if
• choose (when, otherwise)
• trim (where, set)
• foreach

数据库环境准备

-- 动态SQL
USE mybatis;CREATE TABLE IF NOT EXISTS `user` (uid INT NOT NULL AUTO_INCREMENT,uname VARCHAR(50) NOT NULL,upassword VARCHAR(50) NOT NULL,u_nickname VARCHAR(50) NOT NULL,PRIMARY KEY (uid)
);INSERT INTO `user` VALUES (NULL, '赵云', '123456', '常山赵子龙'), (NULL, '张飞', 'asdfff', '燕人张飞'), (NULL, '关羽', 'ghhjkl', '关云长');

sql标签

@Data
@AllArgsConstructor
@NoArgsConstructor
public class User implements Serializable {private static final long serialVersionUID = -5970028623059313270L;private Integer uid;private String uname;private String upassword;private String unickname;
}

public interface UserMapper {User selectByPrimaryKey(Integer uid);
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="org.westos.mapper.UserMapper"><sql id="baseColumn">uid, uname, upassword, u_nickname</sql><select id="selectByPrimaryKey" resultType="User">select<include refid="baseColumn"/>fromuserwhereuid = #{uid}</select>
</mapper>

@Test
public void test() {try (SqlSession sqlSession = SqlSessionFactoryUtil.getSqlSessionFactory().openSession()) {UserMapper mapper = sqlSession.getMapper(UserMapper.class);User user = mapper.selectByPrimaryKey(1);System.out.println(JSON.toJSONString(user, true));} catch (IOException e) {e.printStackTrace();}
}

==>  Preparing: select uid, uname, upassword, u_nickname from user where uid = ?
==> Parameters: 1(Integer)
<==    Columns: uid, uname, upassword, u_nickname
<==        Row: 1, 赵云, 123456, 常山赵子龙
<==      Total: 1
{"uid":1,"uname":"赵云","unickname":"常山赵子龙","upassword":"123456"
}

if标签

//根据条件查询
//使用if标签
List<User> selectAll(User user);

<select id="selectAll" resultType="User">select<include refid="baseColumn"/>fromuserwhere  1 = 1<if test="uid != null">and uid = #{uid}</if><if test="uname != null and uname != ''">and uname = #{uname}</if><if test="upassword != null and upassword != ''">and upassword = #{upassword}</if>
</select>

如果传递的User对象属性都为null，那么就是查询全部User。

@Test
public void test() {try (SqlSession sqlSession = SqlSessionFactoryUtil.getSqlSessionFactory().openSession()) {UserMapper mapper = sqlSession.getMapper(UserMapper.class);User user = new User();user.setUid(2);user.setUname("张飞");List<User> users = mapper.selectAll(user);System.out.println(JSON.toJSONString(users, true));} catch (IOException e) {e.printStackTrace();}
}

User对象的uid和uname是有值的。

==>  Preparing: select uid, uname, upassword, u_nickname from user where 1 = 1 and uid = ? and uname = ?
==> Parameters: 2(Integer), 张飞(String)
<==    Columns: uid, uname, upassword, u_nickname
<==        Row: 2, 张飞, asdfff, 燕人张飞
<==      Total: 1
[{"uid":2,"uname":"张飞","unickname":"燕人张飞","upassword":"asdfff"}
]

where标签

where 元素只会在子元素返回任何内容的情况下才插入 “WHERE” 子句。而且，若子句的开头为 “AND” 或 “OR”，where 元素也会将它们去除。

//根据条件查询
//使用where标签
List<User> selectAll2(User user);

<!--where标签 标签的特点是只有满足if判断条件，才输出where，而且可以去除 第一个 and / or-->
<select id="selectAll2" resultType="User">select<include refid="baseColumn"/>fromuser<where><if test="uid != null">and uid = #{uid}</if><if test="uname != null and uname != ''">and uname = #{uname}</if><if test="upassword != null and upassword != ''">and upassword = #{upassword}</if></where>
</select>

@Test
public void test() {try (SqlSession sqlSession = SqlSessionFactoryUtil.getSqlSessionFactory().openSession()) {UserMapper mapper = sqlSession.getMapper(UserMapper.class);User user = new User();user.setUid(2);user.setUname("张飞");List<User> users = mapper.selectAll2(user);System.out.println(JSON.toJSONString(users, true));} catch (IOException e) {e.printStackTrace();}
}

==>  Preparing: select uid, uname, upassword, u_nickname from user WHERE uid = ? and uname = ?
==> Parameters: 2(Integer), 张飞(String)
<==    Columns: uid, uname, upassword, u_nickname
<==        Row: 2, 张飞, asdfff, 燕人张飞
<==      Total: 1
[{"uid":2,"uname":"张飞","unickname":"燕人张飞","upassword":"asdfff"}
]

@Test
public void test() {try (SqlSession sqlSession = SqlSessionFactoryUtil.getSqlSessionFactory().openSession()) {UserMapper mapper = sqlSession.getMapper(UserMapper.class);User user = new User();user.setUname("");List<User> users = mapper.selectAll2(user);System.out.println(JSON.toJSONString(users, true));} catch (IOException e) {e.printStackTrace();}
}

==>  Preparing: select uid, uname, upassword, u_nickname from user
==> Parameters:
<==    Columns: uid, uname, upassword, u_nickname
<==        Row: 1, 赵云, 123456, 常山赵子龙
<==        Row: 2, 张飞, asdfff, 燕人张飞
<==        Row: 3, 关羽, ghhjkl, 关云长
<==      Total: 3
[{"uid":1,"uname":"赵云","unickname":"常山赵子龙","upassword":"123456"},{"uid":2,"uname":"张飞","unickname":"燕人张飞","upassword":"asdfff"},{"uid":3,"uname":"关羽","unickname":"关云长","upassword":"ghhjkl"}
]

set标签

void updateUser(User user);

<!--set标签-->
<update id="updateUser">updateuser<set><if test="uname != null and uname != ''">uname = #{uname},</if><if test="upassword != null and upassword != ''">upassword = #{upassword},</if><if test="unickname != null and unickname != ''">u_nickname = #{unickname},</if></set>where uid = #{uid}
</update>

@Test
public void test() {try (SqlSession sqlSession = SqlSessionFactoryUtil.getSqlSessionFactory().openSession()) {UserMapper mapper = sqlSession.getMapper(UserMapper.class);User user = new User();user.setUid(3);user.setUname("黄忠");user.setUpassword("huang");user.setUnickname("汉升");mapper.updateUser(user);sqlSession.commit();System.out.println(JSON.toJSONString(mapper.selectByPrimaryKey(3), true));} catch (IOException e) {e.printStackTrace();}
}

==>  Preparing: update user SET uname = ?, upassword = ?, u_nickname = ? where uid = ?
==> Parameters: 黄忠(String), huang(String), 汉升(String), 3(Integer)
<==    Updates: 1
Committing JDBC Connection [com.mysql.jdbc.JDBC4Connection@10a035a0]
==>  Preparing: select uid, uname, upassword, u_nickname from user where uid = ?
==> Parameters: 3(Integer)
<==    Columns: uid, uname, upassword, u_nickname
<==        Row: 3, 黄忠, huang, 汉升
<==      Total: 1
{"uid":3,"uname":"黄忠","unickname":"汉升","upassword":"huang"
}

trim标签

<select id="findActiveBlogLike"resultType="Blog">SELECT * FROM BLOG<where><if test="state != null">state = #{state}</if><if test="title != null">AND title like #{title}</if><if test="author != null and author.name != null">AND author_name like #{author.name}</if></where>
</select>

where 元素只会在子元素返回任何内容的情况下才插入 “WHERE” 子句。而且，若子句的开头为 “AND” 或 “OR”，where 元素也会将它们去除。

如果 where 元素与你期望的不太一样，你也可以通过自定义 trim 元素来定制 where 元素的功能。比如，和 where 元素等价的自定义 trim 元素为：

<trim prefix="WHERE" prefixOverrides="AND |OR ">...
</trim>

查询

//使用trim标签
List<User> selectAll3(User user);

<!--trim标签的属性，prefix增加一个前缀，prefixOverrides去除一个前缀-->
<!--suffix增加一个后缀，suffixOverrides去除一个后缀-->
<!--主要是在插入、更新时使用-->
<select id="selectAll3" resultType="User">select<include refid="baseColumn"/>fromuser<trim prefix="where" prefixOverrides="and |or "><if test="uid != null">and uid = #{uid}</if><if test="uname != null and uname.trim != ''">and uname = #{uname}</if></trim>
</select>

prefix：增加一个前缀

prefixOverrides：去除一个前缀。

@Test
public void test() {try (SqlSession sqlSession = SqlSessionFactoryUtil.getSqlSessionFactory().openSession()) {UserMapper mapper = sqlSession.getMapper(UserMapper.class);User user = new User();user.setUid(3);user.setUname("黄忠");System.out.println(JSON.toJSONString(mapper.selectAll3(user), true));} catch (IOException e) {e.printStackTrace();}
}

==>  Preparing: select uid, uname, upassword, u_nickname from user where uid = ? and uname = ?
==> Parameters: 3(Integer), 黄忠(String)
<==    Columns: uid, uname, upassword, u_nickname
<==        Row: 3, 黄忠, huang, 汉升
<==      Total: 1
[{"uid":3,"uname":"黄忠","unickname":"汉升","upassword":"huang"}
]

更新

//使用trim标签
void updateUser2(User user);

<!--prefix增加一个前缀，suffixOverrides去除一个后缀-->
<update id="updateUser2">updateuser<trim prefix="set" suffixOverrides=","><if test="uname != null and uname.trim != ''">uname = #{uname},</if><if test="upassword != null and upassword.trim != ''">upassword = #{upassword},</if><if test="unickname != null and unickname.trim != ''">u_nickname = #{unickname},</if></trim>where uid = #{uid}
</update>

@Test
public void test() {try (SqlSession sqlSession = SqlSessionFactoryUtil.getSqlSessionFactory().openSession()) {UserMapper mapper = sqlSession.getMapper(UserMapper.class);User user = new User();user.setUid(3);user.setUname("马超");user.setUpassword("ma");user.setUnickname("马儿");mapper.updateUser2(user);sqlSession.commit();System.out.println(JSON.toJSONString(mapper.selectAll3(user), true));} catch (IOException e) {e.printStackTrace();}
}

==>  Preparing: update user set uname = ?, upassword = ?, u_nickname = ? where uid = ?
==> Parameters: 马超(String), ma(String), 马儿(String), 3(Integer)
<==    Updates: 1
Committing JDBC Connection [com.mysql.jdbc.JDBC4Connection@481a15ff]
==>  Preparing: select uid, uname, upassword, u_nickname from user where uid = ? and uname = ?
==> Parameters: 3(Integer), 马超(String)
<==    Columns: uid, uname, upassword, u_nickname
<==        Row: 3, 马超, ma, 马儿
<==      Total: 1
[{"uid":3,"uname":"马超","unickname":"马儿","upassword":"ma"}
]

插入

//使用trim标签
void insertUser(User user);

<insert id="insertUser">insert into user<trim prefix="(" suffix=")" suffixOverrides=","><if test="uname != null">uname,</if><if test="upassword != null">upassword,</if><if test="unickname != null">u_nickname,</if></trim><trim prefix="values (" suffix=")" suffixOverrides=","><if test="uname != null">#{uname},</if><if test="upassword != null">#{upassword},</if><if test="unickname != null">#{unickname},</if></trim>
</insert>

@Test
public void test() {try (SqlSession sqlSession = SqlSessionFactoryUtil.getSqlSessionFactory().openSession()) {UserMapper mapper = sqlSession.getMapper(UserMapper.class);User user = new User();user.setUname("黄忠");user.setUpassword("huang");user.setUnickname("汉升");mapper.insertUser(user);sqlSession.commit();System.out.println(JSON.toJSONString(mapper.selectAll(null), true));} catch (IOException e) {e.printStackTrace();}
}

==>  Preparing: insert into user ( uname, upassword, u_nickname ) values ( ?, ?, ? )
==> Parameters: 黄忠(String), huang(String), 汉升(String)
<==    Updates: 1
Committing JDBC Connection [com.mysql.jdbc.JDBC4Connection@706a04ae]
==>  Preparing: select uid, uname, upassword, u_nickname from user where 1 = 1
==> Parameters:
<==    Columns: uid, uname, upassword, u_nickname
<==        Row: 1, 赵云, 123456, 常山赵子龙
<==        Row: 2, 张飞, asdfff, 燕人张飞
<==        Row: 3, 马超, ma, 马儿
<==        Row: 4, 黄忠, huang, 汉升
<==      Total: 4
[{"uid":1,"uname":"赵云","unickname":"常山赵子龙","upassword":"123456"},{"uid":2,"uname":"张飞","unickname":"燕人张飞","upassword":"asdfff"},{"uid":3,"uname":"马超","unickname":"马儿","upassword":"ma"},{"uid":4,"uname":"黄忠","unickname":"汉升","upassword":"huang"}
]

foreach

动态 SQL 的另一个常见使用场景是对集合进行遍历（尤其是在构建 IN 条件语句的时候）。比如：

//使用foreach标签
List<User> select(List<Integer> list);

<select id="select" resultType="User">select<include refid="baseColumn"/>fromuser<trim prefix="where">uid in <foreach collection="list" open="(" close=")" separator="," index="index" item="item">#{item}</foreach></trim>
</select>

@Test
public void test() {try (SqlSession sqlSession = SqlSessionFactoryUtil.getSqlSessionFactory().openSession()) {UserMapper mapper = sqlSession.getMapper(UserMapper.class);ArrayList<Integer> arrayList = new ArrayList<>(Arrays.asList(1, 2, 3, 4));List<User> users = mapper.select(arrayList);System.out.println(JSON.toJSONString(users, true));} catch (IOException e) {e.printStackTrace();}
}

==>  Preparing: select uid, uname, upassword, u_nickname from user where uid in ( ? , ? , ? , ? )
==> Parameters: 1(Integer), 2(Integer), 3(Integer), 4(Integer)
<==    Columns: uid, uname, upassword, u_nickname
<==        Row: 1, 赵云, 123456, 常山赵子龙
<==        Row: 2, 张飞, asdfff, 燕人张飞
<==        Row: 3, 马超, ma, 马儿
<==        Row: 4, 黄忠, huang, 汉升
<==      Total: 4
[{"uid":1,"uname":"赵云","unickname":"常山赵子龙","upassword":"123456"},{"uid":2,"uname":"张飞","unickname":"燕人张飞","upassword":"asdfff"},{"uid":3,"uname":"马超","unickname":"马儿","upassword":"ma"},{"uid":4,"uname":"黄忠","unickname":"汉升","upassword":"huang"}
]

choose…when…otherwise

//使用choose...when...otherwise标签
User select2(User user);

<select id="select2" resultType="User">select<include refid="baseColumn"/>fromuserwhere<choose><when test="uid != null">uid = #{uid}</when><when test="uname != null">uname = #{uname}</when><otherwise>upassword = #{upassword}</otherwise></choose>
</select>

@Test
public void test() {try (SqlSession sqlSession = SqlSessionFactoryUtil.getSqlSessionFactory().openSession()) {UserMapper mapper = sqlSession.getMapper(UserMapper.class);User user = new User();user.setUid(1);User user1 = mapper.select2(user);System.out.println(JSON.toJSONString(user1, true));} catch (IOException e) {e.printStackTrace();}
}

==>  Preparing: select uid, uname, upassword, u_nickname from user where uid = ?
==> Parameters: 1(Integer)
<==    Columns: uid, uname, upassword, u_nickname
<==        Row: 1, 赵云, 123456, 常山赵子龙
<==      Total: 1
{"uid":1,"uname":"赵云","unickname":"常山赵子龙","upassword":"123456"
}

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