【数据库】SQL语句分析之Union联结查询

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/m18633778874/article/details/84346601

         前言

展示SQL语句

分析SQL语句

  一、union语句

  二、条件语句

  三、查询结果图

小结

---

前言

    小编最近在项目中接触到一个超长的SQL语句,发现里面的语法很丰富,可以拿出来记录一下,当然也有将其中重复的部分进行删减,方便博友们进行查看分析。

展示SQL语句

    SQL语句查询，作用是从两张表 t_card_order 和 t_card_consume_info 中查询出各自需要的字段信息。

      SELECT
            p.pay_time AS paymentCreateTime,
            p.thirdpart_trans_code AS thirdpartTransCode,
            o.id AS orderCode,
            '购卡' AS payType,           
            CASE o.card_type
              WHEN '1' THEN '储值卡'
              WHEN '2' THEN '权益卡'
            END AS cardType
        FROM t_card_order o
          RIGHT JOIN t_order_payment p
            ON o.id = p.order_code
          LEFT JOIN t_movie_cinema c
            ON o.cinema_code = c.cinema_code
        WHERE            
            <if test="param.cinemaCodes != null">
                AND o.cinema_code in
                <foreach item="item" index="index" collection="param.cinemaCodes" open="
                  (" separator="," close=")">
                    #{item}
                </foreach>
            </if>           
            <choose>
                <when test="param.dateTag == 'week'">
                    AND DATE_SUB(CURDATE(), INTERVAL 7 DAY) <= DATE(p.pay_time)
                </when>
                <when test="param.dateTag == 'oneMonth'">
                    AND DATE_SUB(CURDATE(), INTERVAL 1 MONTH) <= DATE(p.pay_time)
                </when>
                <when test="param.dateTag == 'threeMonth'">
                    AND DATE_SUB(CURDATE(), INTERVAL 3 MONTH) <= DATE(p.pay_time)
                </when>
            </choose>
        UNION
        SELECT
            info.create_time AS paymentCreateTime,
            info.transaction_id AS thirdpartTransCode,
            info.id AS orderCode,
            '充值' AS payType,
            CASE info.card_type
              WHEN '1' THEN '储值卡'
              WHEN '2' THEN '权益卡'
            END AS cardType
        FROM t_card_consume_info info
        LEFT JOIN t_movie_cinema c
        ON info.cinema_code = c.cinema_code
        WHERE info.consume_type = 0
            AND consume_comment != '购卡'
            AND consume_comment != '绑卡首次充值' 

分析SQL语句

   一、union语句

    注意：使用union语句，联结的两张表之间的字段个数、名称必须保持一致，否则会报错

     -- 第一张表中的字段如下：
            p.pay_time AS paymentCreateTime,
            p.thirdpart_trans_code AS thirdpartTransCode,
            o.id AS orderCode,
            '购卡' AS payType,           
            CASE o.card_type
              WHEN '1' THEN '储值卡'
              WHEN '2' THEN '权益卡'
            END AS cardType
    -- 第二张表中的字段如下：
            info.create_time AS paymentCreateTime,
            info.transaction_id AS thirdpartTransCode,
            info.id AS orderCode,
            '充值' AS payType,
            CASE info.card_type
              WHEN '1' THEN '储值卡'
              WHEN '2' THEN '权益卡'
            END AS cardType

  二、条件语句

      三种条件语句：case、if和choose whenti

     ---case条件语句 
            CASE o.card_type
              WHEN '1' THEN '储值卡'
              WHEN '2' THEN '权益卡'
            END AS cardType
     ---if条件语句 
            <if test="param.cinemaCodes != null">
                AND o.cinema_code in
                <foreach item="item" index="index" collection="param.cinemaCodes" open="
                  (" separator="," close=")">
                    #{item}
                </foreach>
            </if>    
       ---choose when条件语句 
            <choose>
                <when test="param.dateTag == 'week'">
                    AND DATE_SUB(CURDATE(), INTERVAL 7 DAY) <= DATE(p.pay_time)
                </when>
                <when test="param.dateTag == 'oneMonth'">
                    AND DATE_SUB(CURDATE(), INTERVAL 1 MONTH) <= DATE(p.pay_time)
                </when>
                <when test="param.dateTag == 'threeMonth'">
                    AND DATE_SUB(CURDATE(), INTERVAL 3 MONTH) <= DATE(p.pay_time)
                </when>
            </choose>

三、查询结果图

      字段 为null值： orderCode、cardType等，产生的原因是左右外连接的结果：left join / right join

小结

      通过一番总结，能够让自己对于SQL语句的查询有更深刻的认识，同时也希望对大家有所帮助！

                                                                             感谢您的访问！