逻辑：如果数据库里有，则进行更新操作，如果没有则插入数据。

这里的insert可以不用写字段

merge INTO table_name a using ( SELECT count( field01) co FROM table_name WHERE field01 = '123456' ) b ON ( b.co <> 0 ) 
WHEN MATCHED THEN
UPDATE 
	SET field02 = 'update' 
WHERE
	field01 = '123456' 
	WHEN NOT matched THEN
INSERT
VALUES
	( '123456', 'insert' )

这里事务要自己提交一下，不然就会卡死

    //事务的一些基础信息，如超时时间、隔离级别、传播属性等
    @Resource
    private TransactionDefinition transactionDefinition;

    //事务管理器
    @Resource
    private PlatformTransactionManager platformTransactionManager;
    
    @Override
    public Json saveServiceImpl(Entity entity) {
        //TransactionStatus：事务的一些状态信息，如是否是一个新的事务、是否已被标记为回滚
        TransactionStatus transaction = platformTransactionManager.getTransaction(transactionDefinition);
        try {
            int count = this.baseMapper.saveMapper(entity);
            System.out.println("事务提交前：" + count);
            platformTransactionManager.commit(transaction);
            System.out.println("事务已提交：" + count);
        } catch (Exception e) {
            e.printStackTrace();
            platformTransactionManager.rollback(transaction);
            return Json.error("保存失败" + e.getMessage(), null, 500);
        }
        return Json.success(1);
    }

xml写法（注意：条件字段{field01}不能被更新）

<insert id="saveMapper" parameterType="com.azure.entity.Entity ">
        merge INTO table_name using (SELECT count(1) coun
        FROM table_name
        WHERE field01 = #{field01,jdbcType=VARCHAR}) b ON (b.coun > 0)
        WHEN NOT matched THEN
        INSERT (field01, field02)
        VALUES (#{field02,jdbcType=VARCHAR},
        #{field03,jdbcType=VARCHAR})
        WHEN matched THEN
        UPDATE
        <trim prefix="set" suffixOverrides=",">
            <if test="field02 != null and field02 != ''">
                a.field02 = #{field02,jdbcType=VARCHAR},
            </if>
            <if test="field03 != null and field03 != ''">
                a.field03 = #{field03,jdbcType=VARCHAR},
            </if>
        </trim>
    </insert>