1.if標籤

<select id="selPersons" parameterType="Map" resultMap="PersonResultMap">
        SELECT * FROM person where 1=1

            <if test="name !=null and name != ''">
                and p_name = #{name }
            </if>
            <
 
if test="age != null">
                and p_age > #{age }
            </if>

    </select>

test：判斷條件，格式：屬性名 = 值1 and 引數名=值2…
如果是引數map，屬性名就key
如果條件成立，會將if裡的sql拼接上，如果是第一個，會自動去掉and。

測試一下生成的sql

2. where標籤

寫where 1=1，sql很不美觀
用where標籤會自動判斷是否需要加where

<select id="selPersons"
 
 parameterType="Map" resultMap="PersonResultMap">
        SELECT * FROM person
        <where>
            <if test="name !=null and name != ''">
                and p_name = #{name }
            </if>
            <if test="age != null">
 

                and p_age > #{age }
            </if>
        </where>
    </select>

測試出來的sql，對比一下

3.choose, when, otherwise標籤

<select id="getEmployees" parameterType="Employee" resultMap="baseResultMap">
        select * from employee
        <where>
            <choose>
                <when test="name!=null">
                    and e_name=#{name}
                </when>
                <when test="age!=null">
                    and e_age=#{age}
                </when>
                <otherwise>
                     1=1
                </otherwise>
            </choose>
        </where>
</select>

Ps:不穿透，when中有一個成立，otherwise不執行

4.Set標籤

<update id="updatePerson" parameterType="Person">
        update person
        <set>
            <if test="name != null and name != ''">
                p_name = #{name },
            </if>
            <if test="age != null">
                p_age = #{age },
            </if>
        </set>
        <where>
            p_id = #{id}
        </where>
    </update>

自動判斷是否需要拼接set條件。用於update

5.Foreach標籤

用於批量操作

<insert id="addPersons" parameterType="List">
        insert into person (p_name,p_age,p_hobby) values
        <foreach collection="list" item="person" separator=",">
            (#{person.name},#{person.age},#{person.hobby})
        </foreach>
    </insert>

    <delete id="delPersons" parameterType="List">
        delete from person where p_id in
        <foreach collection="list" item="person" separator="," open="(" close=")">
            #{person.id}
        </foreach>
    </delete>

collection: 集合型別
item：當前項的名字
separator: 分割字元
open：起始字元
close：結束字元