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最近使用Mybatis作为持久层框架，自然/动态sql写得也比较多了，最常见的就是在查询语句中使用if标签来动态地改变过滤条件。<select id="productId" parameterType="Product" resultMap="getProduct">
SELECT * FROM PRODUCT t WHERE (1=1)
<if test="Product.productId!=null">
AND t.PRODUCT_ID = #{Product.productId}
</if>
<if test="Product.productName!=null">
AND t.PRODUCT_NAME like ‘%#{Product.productName}%‘
</if>
<if test="Product.inStock!=null">
AND t.PRODUCT.INSTOCK = 0
</if>
ORDER BY SEQUENCE_NO
</select>
<insert id="addCategory" parameterType="Category">
INSERT INTO CATEGORY(
<if test="categoryName!=null and categoryName!=‘‘ ">
CATEGORY_NAME
</if>
<if test="categoryId!=null and categoryId!=‘‘ ">
CATEGORY_ID
</if>
ADD_TIME)
VALUES(
<if test="categoryName!=null and categoryName!=‘‘ ">
#{categoryName, jdbcType=VARCHAR}
</if>
<if test="categoryId!=null and categoryId!=‘‘ ">
#{categoryId, jdbcType=NUMERIC}
</if>
currentTimeStamp
)
</insert>
<update id="updateCategory" parameterType="Category">
update CATEGORY t SET
<if test="categoryName!=null">
t.CATEGORY_NAME = #{categoryName, jdbcType=VARCHAR},
</if>
<if test="updateUser!=null">
t.UPDATE_USER = #{updateUser, jdbcType=VARCHAR},
</if>
t.UPDATE_TIME = currentTimeStamp
WHERE t.CATEGORY_ID = #{categoryId, jdbcType=NUMERIC}
</update>
<select id="findInStockLike" resultType="Product">
SELECT * FROM PRODUCT WHERE STATE = ‘INSTOCK‘
<choose>
<when>
AND TITLE LIKE #{title}
</when>
<when>
AND BRAND_NAME LIKE #{brand.name}
</when>
<otherwise>
AND INMARKETING = 0
</otherwise>
</choose>
</select>
<select id="findInStockProductLike" resultType="Product">
SELECT * FROM PRODUCT
<when>
<if test="state!=null">
STATE = #{state}
</if>
<if test="title!=null">
AND TITLE LIKE #{title}
</if>
<if test="brand!=null and brand.name!=null">
AND BRAND_NAME LIKE #{brand.name}
</if>
</when>
</select>
在实际应用中，我通常是不写where标签，而在where关键字之后加上1=1的条件。即不管有无动态条件，总可以得到完整的sql：select * from A where 1=1。。。
<update id="updateUserInfo">
UPDATE USER
<set>
<if test="userName!=null">USER_NAME = #{userName}</if>
<if test="userPsw!=null">USER_PSW = #{userPsw}</if>
<if test="userEmail!=null">USER_EMAIL = #{userEmail}</if>
</set>
WHERE USER_ID = #{userId}
</update>
<select id="selectProdcutNoInOrder" resultType="String">
SELECT COUNT(0) FROM PRODUCT a LEFT JOIN ORDER_INFO b
ON a.PRODUCT_ID = b.PRODUCT_ID
WHERE a.STATUS in(‘1‘, ‘2‘, ‘3‘, ‘6‘)
<if test="list.size()>0">
AND b.PHONE_NUM IN
<foreach item="phoneNumList" collection="list" open="(" separator="," close=")">
#{phoneNumList.num}
</foreach>
</if>
</select> foreach 元素的功能是非常强大的，它允许你指定一个集合，声明可以用在元素体内的集合项和索引变量。
<select id="selectProductIn" resultType="com.dowik.dowikmall.Product">
SELECT * FROM PRODUCT p WHERE PRODUCT_ID IN
<foreach item="productIdList" index="index" collection="list" open="(" separetor="," close=")">
#{productIdList}
</foreach>
</select>
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原文地址：http://my.oschina.net/lock0818/blog/506206
