引言
include标签用法是引用sql片段
sql标签 是书写sql片段 被include 引用的

<sql id="query_where">
        <where>
            excel_file_id = #{excelFileId}
            <if test="field1 != null">
                AND field1 = #{field1}
            </if>

            <if test="field2 != null">
                AND field2 = #{field2}
            </if>

            <if test="field3 != null">
                AND field3 = #{field3}
            </if>
        </where>
    </sql>

引用

SELECT * FROM t_excel_file_detail
        <include refid="query_where"></include>