xml映射文件部分内容:
<select id="selectIfCondition" resultType="com.heiketu.testpackage.pojo.Product">
SELECT
prod_id prodId,
vend_id vendId,
prod_name prodName,
prod_desc prodDesc
FROM Products <where>
<if test="prodId != null and prodId != ''">
AND prod_id = #{prodId} </if>
<if test="prodName != null and prodName != ''">
AND prod_name = #{prodName} </if>
</where></select>映射文件中的where标签可以过滤掉条件语句中的第一个and或or关键字。以上SQL当prodId!=null时会被mybatis的where标签去除掉多余的and关键字，生成的sql如下:
SELECT prod_id prodId, vend_id vendId, prod_name prodName, prod_desc prodDescFROM ProductsWHERE prod_id = ?AND prod_name = ?
注: where标签只能去除第一个条件中出现的前置 and 关键字。像以下情况，where就无能为力了:
<select id="selectIfCondition" resultType="com.heiketu.testpackage.pojo.Product">
SELECT
prod_id prodId,
vend_id vendId,
prod_name prodName,
prod_desc prodDesc
FROM Products <where>
<if test="prodId != null and prodId != ''">
prod_id = #{prodId} AND </if>
<if test="prodName != null and prodName != ''">
prod_name = #{prodName} AND </if>
</where></select>Mybatis会把次SQL语句拼接成:
SELECT prod_id prodId, vend_id vendId, prod_name prodName, prod_desc prodDescFROM ProductsWHERE prod_id = ? AND prod_name = ? AND
where无法去除掉后面的and关键字，此时sql语句出现语法错误。
要想解决以上where标签无法处理的问题，可以考虑使用trim标签。