mybatis from entry to the master (D) Dynamic SQL
A Dian Profile
One of the powerful features Mybatis is dynamic SQL, it can dynamically splicing sql statement, reduce the workload of development.
Mybatis dynamic sql following four types of tags
1. if
2. choose (when, otherwise)
3. trim (where, set)
4. foreach
Two Dian <if />
<if /> tag corresponds to an if statement java language, by determining whether the preset condition to splice sql statement. Wherein the determination conditions described ongl expressions, such as e.method (args) calling the object method, e.property access object properties, ongl can refer to this blog
DynamicSqlUserMapper.java
User selectUserByUserNameAndAge(@Param("userName") String userName, @Param("age") Integer age);
DynamicSqlUserMapper.xml
<select id="selectUserByUserNameAndAge" resultMap="BaseResultMap"> select * from user where user_name=#{userName} <if test="age != null"> and age=#{age} </if> </select>
DynamicSqlTests.java
@Test public void ifTest(){ // 测试if动态sql User user=dynamicSqlUserMapper.selectUserByUserNameAndAge("tim", null); Assert.assertTrue(user!= null); user=dynamicSqlUserMapper.selectUserByUserNameAndAge("tim", 999); Assert.assertTrue(user == null); }
Wed and <choose />
<Choose /> similar to java in if ... else if ... else ... statement
DynamicSqlUserMapper.java
List<User> selectUserLikeByChoose(@Param("userName") String userName, @Param("age") Integer age, @Param("country") String country);
DynamicSqlUserMapper.xml
<!-- 2. choose(when, otherwise) 类似于 if, else --> <select id="selectUserLikeByChoose" resultMap="BaseResultMap"> select * from user where user_name like '${userName}' <choose> <when test="age != null"> and age = #{age} </when> <when test="country != null "> and country like '${country}' </when> <otherwise> and user_id > 10 </otherwise> </choose> </select>
DynamicSqlTests.java
@Test public void chooseTest () { // Test choose dynamic SQL List <the User> Users = dynamicSqlUserMapper.selectUserLikeByChoose ( "% T",. 11, null ); // Note that, like xx%, where% of control parameters into sqlSession .commit (); Assert.assertTrue (the Users =! null ); Assert.assertTrue (users.size () == 1 ); the Users = dynamicSqlUserMapper.selectUserLikeByChoose ( "% t", null , "% State"); // Note that, like xx%, where% of control parameters into sqlSession.commit (); Assert.assertTrue (Users =! null ); Assert.assertTrue(users.size()>1); }
Four Dian <where />
<Where /> tag within the tag is that when the condition is not satisfied, it will remove inappropriate sql keywords. Such as <where /> inside of a condition does not hold, then where keywords will not be added stitching, <where /> on the inside of the conditions is not satisfied, the second holds, the second condition in the sql remove the statement "and" keyword. <Where /> and <set /> actually <trim /> the particular implementation.
DynamicSqlUserMapper.java
List<User> selectUserLikeByWhere(@Param("userName") String userName, @Param("country") String country);
DynamicSqlUserMapper.xml
<!-- 3. trim(where, set) -->
<select id="selectUserLikeByWhere" resultMap="BaseResultMap">
select * from user
<where>
<if test="userName !=null and userName.trim().length()>0">
user_name like #{userName}
</if>
<if test="country !=null and country.trim().length()>0">
and country like #{country}
</if>
</where>
</select>
DynamicSqlTests.java
@Test public void whereTest () { // use where elements List <the User> Users = dynamicSqlUserMapper.selectUserLikeByWhere ( "% T", ""); // Country parameter spaces are filtered, select * from user WHERE user_name like ? Assert.assertTrue (the Users =! null ); Assert.assertTrue (users.size () > 1 ); // ? the SELECT * from the User Country like the WHERE // The first is null, splicing, will remove the second qualified and, after re-stitching the Users = dynamicSqlUserMapper.selectUserLikeByWhere ( null , "China" ); Assert.assertTrue (the Users ! = null ); Assert.assertTrue (users.size () == 1 ); // the SELECT * from the User // conditions are not in line with, and will not splice sql, "where" keyword was not spliced the Users = dynamicSqlUserMapper.selectUserLikeByWhere ( null , null ); Assert.assertTrue (Users =! null ); Assert.assertTrue (users.size () ==. 5 ); }
Five Dian <trim />
By the above, <where /> tag is <trim /> tag particular implementation. Sql prefix for attribute within the tag before adding the prefix, represents prefixOverrides attribute values will be used to replace the corresponding prefix in the prefix property <trim />.
In addition, there suffix suffix property, suffixOverrides rewrite value.
Below we will replace the label <trim /> <where />.
DynamicSqlUserMapper.java
List<User> selectUserLikeByTrim(@Param("userName") String userName, @Param("country") String country);
DynamicSqlUserMapper.xml
<! - using custom trim element method, replacing WHERE -> < SELECT ID = "selectUserLikeByTrim" The resultMap = "BaseResultMap" > SELECT * from User < trim prefix = "WHERE" prefixOverrides = "the AND | OR" > <! - prefix characters replace the character corresponding prefixOverrides, prefixOverrides spaces is necessary -> < IF Test = "null and userName.trim the userName = () length ()> 0!." > USER_NAME like the userName # {} </ IF > < IF Test = "Country!=null and country.trim().length()>0"> and country like #{country} </if> </trim> </select>
DynamicSqlTests.java
@Test public void trimTest () { // use trim elements, and where to achieve the same effect List <the User> Users = dynamicSqlUserMapper.selectUserLikeByTrim ( "% T", ""); // Country parameter space will be filtered, ? the SELECT * from the User the WHERE user_name like Assert.assertTrue (! the Users = null ); Assert.assertTrue (users.size () > 1 ); // ? the SELECT * from the User Country like the WHERE // The first is null, stitching when will remove the second qualifying and, after re-stitching the Users = dynamicSqlUserMapper.selectUserLikeByTrim ( null , "China" ); Assert.assertTrue (the Users ! = null); Assert.assertTrue (users.size () == 1 ); // the SELECT * from the User // conditions are not in line with, and will not splice sql, "where" keyword was not spliced the Users = dynamicSqlUserMapper.selectUserLikeByTrim ( null , null ); Assert.assertTrue (Users =! null ); Assert.assertTrue (users.size () ==. 5 ); }
Six Dian <foreach />
<Foreach /> tag corresponds java for statement, indicating the need to traverse Collection attribute collection object, index when the index attribute indicates traversal, item property indicates the object obtained when the traversal,
It is used as follows
DynamicSqlUserMapper.java
List<User> selectUserByUserIdsByForeach(@Param("userIds") List<Integer> userIds);
DynamicSqlUserMapper.xml
<!-- 4. foreach --> <!-- select * from user where user_id in ( ? , ? , ? ) --> <select id="selectUserByUserIdsByForeach" resultMap="BaseResultMap"> select * from user where user_id in <foreach collection="userIds" index="index" item="item" open="(" separator="," close=")"> <!-- 注意，当collection为map时，index为key, item为value --> #{item} </foreach> </select>
DynamicSqlTests.java
@Test public void foreachTest(){ List<Integer> userIds= Arrays.asList(1,2,3); // select * from user where user_id in ( ? , ? , ? ) List<User> users=dynamicSqlUserMapper.selectUserByUserIdsByForeach(userIds); Assert.assertTrue(users!=null); Assert.assertTrue(users.size()==3); }
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Learning materials:
Official Documentation - Dynamic SQL