learn from：http://www.mybatis.org/mybatis-3/dynamic-sql.html
mybatis支持动态拼接sql语句。主要有：
- if
- choose (when, otherwise)
- trim (where, set)
- foreach
1.if
首先看基本实例：
<select id="findActiveBlogWithNameLike" resultType="Blog"> SELECT * FROM blog WHERE state = 'active' <if test="name != null"> AND name LIKE #{name} </if> </select>
List<Blog> findActiveBlogWithNameLike(String name);
这里遇到一个问题：
There is no getter for property named 'name' in 'class java.lang.String'###
Cause: org.apache.ibatis.reflection.ReflectionException: There is no getter for property named 'name' in 'class java.lang.String'
也就是说，mybatis将name当做输入参数的一个属性，并且期望通过getter方法来获取它的值。很容易想到，将输入参数改成Blog就可以了。
然而，这并不符合我们的查询习惯，比如，如果是Blog就必须这样查询：
@Test public void testFindActiveBlogWithNameLike() throws Exception{ Blog key = new Blog(); key.setName("test%"); List<Blog> blogs = mapper.findActiveBlogWithNameLike(key); System.out.println(blogs); return; }
为了一个String字段而创建一个类，看着要多别扭有多别扭。当然，前提是我们仅仅是查询name条件，所以会觉得其他属性冗余。如果是多条件查询，那么Blog必然是最好的选择。那么，仅仅传入String的话应该也是可以的。参考1,参考2。
第一种做法是简单类型都是使用_parameter来代替。
<select id="findActiveBlogWithNameLikeByString" parameterType="java.lang.String" resultType="Blog"> SELECT * FROM blog WHERE name LIKE '%${_parameter}%' </select>
第二种做法比较容易理解,在方法参数前添加@Param(value="xxx")注解来使用xxx作为传入参数。
<select id="findActiveBlogWithNameLikeByString" parameterType="java.lang.String" resultType="Blog"> SELECT * FROM blog WHERE name LIKE '%${key}%' </select> List<Blog> findActiveBlogWithNameLikeByString(@Param(value = "key") String key);
两种做法均可，所以，看你喜欢了，是想要省事简洁还是通俗易读。在这里，还是选择第0种方案，即传入Blog对象来作为查询条件。
下面简单介绍if的语法：
- if节点中，属性test是一个boolean值，为true的时候将拼接if里的sql语句。
- 参数值是可以包含一些掩码或通配符的.比如通配符%和占位符_
所以，很简单很容易理解。
场景一： 查询blog的名字name like %Insert and 作者author的username like Ryan%.
首先看期望的结果，blog表中有三条满足name like:
mysql> select * from blog; +----+------------+-----------+--------------+--------+ | id | name | author_id | co_author_id | state | +----+------------+-----------+--------------+--------+ | 1 | test | 3 | 4 | active | | 8 | testInsert | 4 | 5 | active | | 9 | testInsert | 5 | 6 | active | | 10 | testInsert | 6 | 7 | active | | 12 | testA | 50 | NULL | active | | 13 | testA | 51 | NULL | active | | 14 | testInsert | NULL | NULL | active | +----+------------+-----------+--------------+--------+ 7 rows in set
这三条中，满足author的username like的有两条：
mysql> select author.id, author.username from author where id in (4,5,6); +----+----------+ | id | username | +----+----------+ | 4 | Ryan | | 5 | Ryan0 | | 6 | Leslie | +----+----------+ 3 rows in set
也就是我们最终希望结果是blog id为8 和 9。
mybatis的sql语句如下：
<select id="findActiveBlogLike" resultType="Blog"> SELECT * FROM blog b, author a WHERE state = 'active' <if test="name != null"> AND name LIKE #{name} </if> AND b.author_id = a.id <if test="author != null and author.name != null"> AND a.username LIKE #{author.username} </if> </select>
当blog的name不为null的时候查询name匹配，当author的username不为null的时候，查询author的username匹配。
- 第一个if节点的test为name，这个会查找Blog的name字段，如果传入参数Blog没有name字段，那么就会像我们开始那样报错。所以，name必须是blog的一个字段。同理，#{name}这个也要和blog字段字面量的值匹配。
- 第二个if节点的test里看到了and，and就是并且。首先判断author是否为null，就是判断Blog对象的author属性是否为null。接着判断author.name是否为null，这里就有点问题了。因为我的Author类中并没有name字段，对应的字段字面量是username。也就是说这里应该是author.username。但我粗心写成了author.name（所以为每段代码编写unit test是多么的重要）。更奇葩的是，这条test通过了判断为真，这里先不讲，后面测试的时候再分析原因。不过这里一定要改成author.username才是正确的做法。
对应的java接口：
List<Blog> findActiveBlogLike(Blog blog);
下面开始测试：
@Test public void testFindActiveBlogLike() throws Exception{ Blog blog = new Blog(); blog.setName("%Insert"); Author author = new Author(); author.setUsername("Ryan%"); blog.setAuthor(author); List<Blog> blogs = mapper.findActiveBlogLike(blog); System.out.println(blogs); assertTrue(blogs.size()==2); return; }
先看结果对不对：
2016-08-06 16:20:19,264 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Opening JDBC Connection 2016-08-06 16:20:19,740 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Created connection 1150284200. 2016-08-06 16:20:19,742 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@448ff1a8] 2016-08-06 16:20:19,745 DEBUG [com.test.mapper.dao.BlogMapper.findActiveBlogLike] - ==> Preparing: SELECT * FROM blog b, author a WHERE state = 'active' AND name LIKE ? AND b.author_id = a.id AND a.username LIKE ? 2016-08-06 16:20:19,888 DEBUG [com.test.mapper.dao.BlogMapper.findActiveBlogLike] - ==> Parameters: %Insert(String), Ryan%(String) 2016-08-06 16:20:19,978 DEBUG [com.test.mapper.dao.BlogMapper.findActiveBlogLike] - <== Total: 2 [Blog{id=8, name='testInsert', author=null, coAuthor=null, posts=null, state='active'}, Blog{id=9, name='testInsert', author=null, coAuthor=null, posts=null, state='active'}]
test通过了，blog也确实是我们想要的两条。但仔细观察结果就会发现几个问题。第一个问题是author为null，这个我们等下再解决。第二问题是sql查询语句查询了author.username like，也就是说我们第二个if节点的test 为true。难道出了问题？我们的Author类明明没有name字段。所以，这里要跟踪下代码。
好吧，跟踪了半天一直到ognl内部，还是没追踪到为什么name翻译成username了。下面还是搞定第一个问题，author为null。
查询的结果映射到Blog，但blog的author字段并没有初始化。很容易就猜测到结果集的字段和blog的author不匹配。这个就用到resultMap而不是resultType。在上一遍博文中记录了下来。
<select id="findBlogMap" resultMap="findBlogResultMap"> SELECT b.id AS id, b.name AS name, b.state AS state, a.id as author_id, a.username as author_username, a.password as author_password, a.email as author_email, a.bio as author_bio FROM blog b, author a WHERE state = 'active' <if test="name != null"> AND name LIKE #{name} </if> AND b.author_id = a.id <if test="author != null and author.username != null"> AND a.username LIKE #{author.username} </if> </select> <resultMap id="findBlogResultMap" type="Blog"> <id property="id" column="id"/> <result property="name" column="name"/> <result property="state" column="state"/> <association property="author" column="author_id" javaType="Author"> <id property="id" column="author_id"/> <result property="username" column="author_username"/> <result property="password" column="author_password"/> <result property="email" column="author_email"/> <result property="bio" column="author_bio"/> </association> </resultMap>
这样测试结果：
2016-08-15 22:42:10,994 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Opening JDBC Connection 2016-08-15 22:42:11,567 DEBUG [org.apache.ibatis.datasource.pooled.PooledDataSource] - Created connection 1627428162. 2016-08-15 22:42:11,569 DEBUG [org.apache.ibatis.transaction.jdbc.JdbcTransaction] - Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@61009542] 2016-08-15 22:42:11,573 DEBUG [com.test.mapper.dao.BlogMapper.findBlogMap] - ==> Preparing: SELECT b.id AS id, b.name AS name, b.state AS state, a.id as author_id, a.username as author_username, a.password as author_password, a.email as author_email, a.bio as author_bio FROM blog b, author a WHERE state = 'active' AND name LIKE ? AND b.author_id = a.id AND a.username LIKE ? 2016-08-15 22:42:11,674 DEBUG [com.test.mapper.dao.BlogMapper.findBlogMap] - ==> Parameters: %Insert(String), Ryan%(String) 2016-08-15 22:42:11,725 DEBUG [com.test.mapper.dao.BlogMapper.findBlogMap] - <== Total: 2 [Blog{id=8, name='testInsert', author=Author{id=4, username='Ryan', password='123456', email='qweqwe@qq.com', bio='this is a blog'}, coAuthor=null, posts=null, state='active'}, Blog{id=9, name='testInsert', author=Author{id=5, username='Ryan0', password='123456', email='qweqwe@qq.com', bio='this is a blog'}, coAuthor=null, posts=null, state='active'}]
2.choose
唯有不断学习方能改变！ -- Ryan Miao
