MyBatis 动态SQL

动态SQL，主要用于解决查询条件不确定的情况。

例如：

select id,name,age,score from student where id=？name=？
age=？score=？

每一个条件都可有可无，那where条件有4*3*2*1=24这么多情况，总不能写24个sql。

可使用动态SQL来解决这一的问题。

动态SQL，即通过MyBatis提供的各种标签对条件作出判断以实现动态拼接SQL语句。这里条件判断使用的表达式为OGNL表达式（Object
Graphic Navigation Language 对象图导航语言）

1、if

<select id="selectByIf" resultType="Student">
select id,name,age,score
from student
where 1 = 1
<if test="name != null and name != ''">
and name like '%' #{name} '%'
</if>
<if test="age > 0">
and age > #{age}
</if>
</select>

if标签，where条件要加 1=1.对于数据量大的查询而言，1=1也是判断条件，还是会影响查询效率。
2、where

<select id="selectByWhere" resultType="Student">
select id,name,age,score
from student
<where>
<if test="name != null and name != ''">
and name like '%' #{name} '%'
</if>
<if test="age > 0">
and age > #{age}
</if>
</where>
</select>

where标签，会自动去掉第一个and

3、choose

<select id="selectByChoose" resultType="Student">
select id,name,age,score
from student
<where>
<choose>
<when test="name != null and name != ''">
and name like '%' #{name} '%'
</when>
<when test="age > 0">
and age > #{age}
</when>
<otherwise>
1 = 2
</otherwise>
</choose>
</where>
</select>

choose标签，类似java里面的switch，when对应case，otherwise对应default。

4、foreach

<select id="selectByForeach" resultType="Student">
select id,name,age,score
from student
<if test="array.length > 0">
where id in
<foreach collection="array" item="myid" open="(" close=")" separator=",">
#{myid}
</foreach>
</if>
</select>

<select id="selectByForeach2" resultType="Student">
select id,name,age,score
from student
<if test="list.size > 0">
where id in
<foreach collection="list" item="myid" open="(" close=")" separator=",">
#{myid}
</foreach>
</if>
</select>

<select id="selectByForeach3" resultType="Student">
select id,name,age,score
from student
<if test="list.size > 0">
where id in
<foreach collection="list" item="myid" open="(" close=")" separator=",">
#{myid.id}
</foreach>
</if>
</select>

foreach针对数组、List等集合形式的数据。

5、include

<select id="selectBySqlFragment" resultType="Student">
<include refid="selectFront" />
from student
<if test="list.size > 0">
where id in
<foreach collection="list" item="myid" open="(" close=")" separator=",">
#{myid.id}
</foreach>
</if>
</select>

<!-- sql片段可以替换Sql任何部分 -->
<sql id="selectFront">
select id,name,age,score
</sql>

sql片段可以提高代码的复用性，但是降低了代码的可读性。

6、mapper对应的Dao接口

@Test
public void test01() {
//        Student stu = new Student("张", 23, 89);
//        Student stu = new Student("", 23, 89);
Student stu = new Student("", 0, 89);

List<Student> students = dao.selectByIf(stu);
for (Student student : students) {
System.out.println(student);
}
}

@Test
public void test02() {
Student stu = new Student("张", 23, 89);
//        Student stu = new Student("", 23, 89);
//        Student stu = new Student("", 0, 89);

List<Student> students = dao.selectByWhere(stu);
for (Student student : students) {
System.out.println(student);
}
}

@Test
public void test03() {
//        Student stu = new Student("张", 23, 89);
//        Student stu = new Student("", 23, 89);
Student stu = new Student("", 0, 89);

List<Student> students = dao.selectByChoose(stu);
for (Student student : students) {
System.out.println(student);
}
}

@Test
public void test04() {
int[] ids = { 1, 2, 3, 7 };

List<Student> students = dao.selectByForeach(ids);
for (Student student : students) {
System.out.println(student);
}
}

@Test
public void test05() {
List<Integer> ids = new ArrayList<Integer>();
ids.add(1);
ids.add(2);
ids.add(3);
ids.add(7);

List<Student> students = dao.selectByForeach2(ids);
for (Student student : students) {
System.out.println(student);
}
}

@Test
public void test06() {
List<Student> stus = new ArrayList<>();
Student stu1 = new Student();
stu1.setId(1);
Student stu2 = new Student();
stu2.setId(2);
Student stu3 = new Student();
stu3.setId(7);

stus.add(stu1);
stus.add(stu2);
stus.add(stu3);

List<Student> students = dao.selectByForeach3(stus);
for (Student student : students) {
System.out.println(student);
}
}

@Test
public void test07() {
List<Student> stus = new ArrayList<>();
Student stu1 = new Student();
stu1.setId(1);
Student stu2 = new Student();
stu2.setId(2);
Student stu3 = new Student();
stu3.setId(7);

stus.add(stu1);
stus.add(stu2);
stus.add(stu3);

List<Student> students = dao.selectBySqlFragment(stus);
for (Student student : students) {
System.out.println(student);
}
}