标签：col 部分 tst odi bsp close NPU 一半 逗号
MyBatis 的强大特性之一便是它的动态 SQL。如果你有使用 JDBC 或其它类似框架的经验，你就能体会到根据不同条件拼接 SQL 语句的痛苦。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 这一特性可以彻底摆脱这种痛苦。
虽然在以前使用动态 SQL 并非一件易事，但正是 MyBatis 提供了可以被用在任意 SQL 映射语句中的强大的动态 SQL 语言得以改进这种情形。
动态 SQL 元素和 JSTL 或基于类似 XML 的文本处理器相似。在 MyBatis 之前的版本中，有很多元素需要花时间了解。MyBatis 3 大大精简了元素种类，现在只需学习原来一半的元素便可。MyBatis 采用功能强大的基于 OGNL 的表达式来淘汰其它大部分元素。
1.if语句
动态 SQL 通常要做的事情是根据条件包含 where 子句的一部分。
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"> <mapper namespace="com.sxt.dao.UserMapper"> <select id="query" resultType="user" parameterType="user"> select * from t_user where 1=1 <if test="name!=null"> and name=#{name} </if> <if test="age>0"> and age=#{age} </if> <if test="id!=null and id>0"> and id=#{id} </if> </select>
测试
@Test public void test1() throws IOException { InputStream in = Resources.getResourceAsStream("mybatis-cfg.xml"); SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in); SqlSession session = factory.openSession(); UserMapper dao = session.getMapper(UserMapper.class); User user = new User(); user.setName("王五"); user.setId(2); user.setAge(18); List<User> list = dao.query(user); for (User user2 : list) { System.out.println(user2); } session.close(); }
结果
2.where语句
<select id="query1" resultType="user" parameterType="user"> select * from t_user <where> <if test="name!=null"> and name=#{name} </if> <if test="age>0"> and age=#{age} </if> <if test="id!=null and id>0"> and id=#{id} </if> </where> </select>
测试
@Test public void test2() throws IOException { InputStream in = Resources.getResourceAsStream("mybatis-cfg.xml"); SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in); SqlSession session = factory.openSession(); UserMapper dao = session.getMapper(UserMapper.class); User user = new User(); // user.setName("王五"); // user.setId(2); // user.setAge(18); List<User> list = dao.query1(user); for (User user2 : list) { System.out.println(user2); } session.close(); }
结果
3.choose语句
<!-- 首先判断name是否为空 如果不为空根据name查询 结束 如果name为空 判断age是否传达 ，不为空大于0根据age查询 否则根据id倒叙 --> <select id="query2" resultType="user" parameterType="user"> select * from t_user where 1=1 <choose> <when test="name!=null"> and name=#{name} </when> <when test="age!=null and age>0"> and age=#{age} </when> <otherwise> order by desc </otherwise> </choose> </select>
测试
@Test public void test3() throws IOException { InputStream in = Resources.getResourceAsStream("mybatis-cfg.xml"); SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in); SqlSession session = factory.openSession(true); UserMapper dao = session.getMapper(UserMapper.class); User user = new User(); // user.setName("王五"); // user.setAge(18); List<User> list = dao.query2(user); for (User user2 : list) { System.out.println(user2); } session.close(); }
结果
4.set语句
<!-- set标签会截取最后一个"," --> <update id="updateUser2" parameterType="user"> update t_user <set> <if test="name!=null"> name=#{name}, </if> <if test="age!=null and age>0"> age=#{age}, </if> </set> where id=#{id} </update>
测试
@Test public void test4() throws IOException { InputStream in = Resources.getResourceAsStream("mybatis-cfg.xml"); SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in); SqlSession session = factory.openSession(true); UserMapper dao = session.getMapper(UserMapper.class); User user = new User(); user.setId(3); user.setName("渣渣"); user.setAge(1); dao.updateUser2(user); session.close(); }
结果
5.trim语句
trim标记是一个格式化的标记，可以完成set或者是where标记的功能
<select id="query3" resultType="user" parameterType="user"> select * from t_user <trim prefix="where" prefixOverrides="and"> <if test="name!=null"> and name=#{name} </if> <if test="age>0"> and age=#{age} </if> <if test="id!=null and id>0"> and id=#{id} </if> </trim> </select>
测试
@Test public void test5() throws IOException { InputStream in = Resources.getResourceAsStream("mybatis-cfg.xml"); SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in); SqlSession session = factory.openSession(); UserMapper dao = session.getMapper(UserMapper.class); User user = new User(); // user.setName("王五"); // user.setId(2); // user.setAge(18); List<User> list = dao.query3(user); for (User user2 : list) { System.out.println(user2); } session.close(); }
结果
6.update语句
<update id="updateUser3" parameterType="user"> update t_user <trim prefix="set" suffixOverrides=","> <if test="name!=null"> name=#{name}, </if> <if test="age!=null and age>0"> age=#{age}, </if> </trim> where id=#{id} </update>
测试
@Test public void test6() throws IOException { InputStream in = Resources.getResourceAsStream("mybatis-cfg.xml"); SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in); SqlSession session = factory.openSession(true); UserMapper dao = session.getMapper(UserMapper.class); User user = new User(); user.setName("花花"); user.setId(5); // user.setAge(1); Integer count = dao.updateUser3(user); System.out.println(count); session.close(); }
结果
7.sql块
sql片段一般用来定义sql中的列
<sql id="baseSql"> id,name,age </sql> <select id="query4" resultType="user" parameterType="user"> select <include refid="baseSql"></include> from t_user <trim prefix="where" prefixOverrides="and"> <if test="name!=null"> and name=#{name} </if> <if test="age>0"> and age=#{age} </if> <if test="id!=null and id>0"> and id=#{id} </if> </trim> </select>
测试
@Test public void test7() throws IOException { InputStream in = Resources.getResourceAsStream("mybatis-cfg.xml"); SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in); SqlSession session = factory.openSession(); UserMapper dao = session.getMapper(UserMapper.class); User user = new User(); // user.setName("王五"); // user.setId(2); // user.setAge(18); List<User> list = dao.query4(user); for (User user2 : list) { System.out.println(user2); } session.close(); }
结果
标签：col 部分 tst odi bsp close NPU 一半 逗号
原文地址：https://www.cnblogs.com/wangdh666/p/11182352.html
