Mybatis中where的用法（动态SQL之where标签）

本文转自：/u014268482/article/details/80623857  仅供自己学习使用

xml映射文件部分内容:

<select  resultType="">
    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 prodDesc
FROM Products
WHERE
    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  resultType="">
    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 prodDesc
FROM Products
WHERE
    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。

可以访问  /qq_24753293/article/details/108533053 查看