Oracle數據庫merge into的使用，存在則更新，不存在則插入

一、在實際應用場景中，咱們會用到：若是這條數據在表中，就更新數據；若是不存在這條數據，就插入這條數據。
在oracle中，能夠使用merge into實現，在mysql中能夠使用ON DUPLICATE KEY UPDATE，這裏只介紹oracle中的merge into實現方法，sql語法以下：

merge into testtable t using dual on (t.id = '1')  
         when not matched then insert (id,a,b) values (1,2,1)  
             when matched then update set b = b+1;  

select * from testtable;（若是testtable是空表，執行上述語句以後以下：） 
id    A    B
--------------- 
1     2    1

select * from testtable;（若是再執行一次merge into,testtable結果）
id    A    B
---------------
1     2    2

解釋：

using dual on (...)是一種固定寫法，也能夠寫成 using (select 1 from dual) on (...) , 表名dual也能夠寫成其餘的名字。

on(...)裏面是條件，能夠寫一個條件，也能夠用and連多個條件。

二、順便介紹一下mybatis中怎麼使用merge into，直接上代碼，代碼使用的是動態sql。

<update id="updateStaffEvaluation" parameterType="java.util.HashMap">  
    merge into staff_evaluation t   
        using dual on (t.login_id = #{loginId,jdbcType=VARCHAR})  
        when not matched then insert  
    <trim prefix="(" suffix=")" suffixOverrides="," >  
        <if test="userId != null" >  
            USER_ID,  
        </if>  
        <if test="loginId != null" >  
            LOGIN_ID,  
        </if>  
        <if test="userName != null" >  
            USER_NAME,  
        </if>  
        <if test="complexScore != null" >  
            COMPLEX_SCORE,  
        </if>  
        <if test="mobileServiceScore != null" >  
            MOBILE_SERVICE_SCORE,  
        </if>  
        <if test="broadbandScore != null" >  
            BROADBAND_SCORE,  
        </if>  
        <if test="keepScore != null" >  
            KEEP_SCORE,  
        </if>  
        <if test="state != null" >  
            STATE,  
        </if>  
        <if test="evaluateText != null" >  
            EVALUATE_TEXT,  
        </if>  
        <if test="beginTime != null" >  
            BEGIN_TIME,  
        </if>  
        <if test="endTime != null" >  
            END_TIME,  
        </if>  
        <if test="evaluatePerson != null" >  
            EVALUATE_PERSON,  
        </if>  
    </trim>  
    <trim prefix="values (" suffix=")" suffixOverrides="," >  
        <if test="userId != null" >  
            #{userId,jdbcType=VARCHAR},  
        </if>  
        <if test="loginId != null" >  
            #{loginId,jdbcType=VARCHAR},  
        </if>  
        <if test="userName != null" >  
            #{userName,jdbcType=VARCHAR},  
        </if>  
        <if test="complexScore != null" >  
            #{complexScore,jdbcType=VARCHAR},  
        </if>  
        <if test="mobileServiceScore != null" >  
            #{mobileServiceScore,jdbcType=VARCHAR},  
        </if>  
        <if test="broadbandScore != null" >  
            #{broadbandScore,jdbcType=VARCHAR},  
        </if>  
        <if test="keepScore != null" >  
            #{keepScore,jdbcType=VARCHAR},  
        </if>  
        <if test="state != null" >  
            #{state,jdbcType=VARCHAR},  
        </if>  
        <if test="evaluateText != null" >  
            #{evaluateText,jdbcType=VARCHAR},  
        </if>  
        <if test="beginTime != null" >  
            #{beginTime,jdbcType=VARCHAR},  
        </if>  
        <if test="endTime != null" >  
            #{endTime,jdbcType=VARCHAR},  
        </if>  
        <if test="evaluatePerson != null" >  
            #{evaluatePerson,jdbcType=VARCHAR},  
        </if>  
    </trim>  
    when matched then update  
    <set>  
        <if test="complexScore != null" >  
            t.COMPLEX_SCORE = #{complexScore,jdbcType=VARCHAR},  
        </if>  
        <if test="mobileServiceScore != null" >  
            t.MOBILE_SERVICE_SCORE = #{mobileServiceScore,jdbcType=VARCHAR},  
        </if>  
        <if test="broadbandScore != null" >  
            t.BROADBAND_SCORE = #{broadbandScore,jdbcType=VARCHAR},  
        </if>  
        <if test="keepScore != null" >  
            t.KEEP_SCORE = #{keepScore,jdbcType=VARCHAR},  
        </if>  
        <if test="evaluateText != null" >  
            t.EVALUATE_TEXT = #{evaluateText,jdbcType=VARCHAR},  
        </if>  
        <if test="beginTime != null" >  
            t.BEGIN_TIME = #{beginTime,jdbcType=VARCHAR},  
        </if>  
        <if test="endTime != null" >  
            t.END_TIME = #{endTime,jdbcType=VARCHAR},  
        </if>  
        <if test="evaluatePerson != null" >  
            t.EVALUATE_PERSON = #{evaluatePerson,jdbcType=VARCHAR},  
        </if>  
    </set>  
    where t.LOGIN_ID = #{loginId,jdbcType=VARCHAR}  
</update>

注意：必定要用update標籤。