Question

我对MYSQL全新，所以对这个问题表示道歉，但我不知道从哪里开始。

我有一个类似于下面的mysql表;

TABLE_1

Record_ID   Price   Price_Difference    New_Value
1           100     null
2           101     1
3           115     14
4           135     20
5           95      -40
6           80      -15

我想要做的是添加和输入null的记录1，因为没有价格差异。

对于记录2，我希望'New_Value'为14的价格差值除以前一记录1中的价格值。

e.g。 14 / @ VAR_1

哪里

@var_1 = (case 
when price_1 < 50 then 0.5
when price_1 between 50.001 and 100 then 1
when price_1 between 100.001 and 150 then 1.25
when price_1 between 150.001 and 150 then  1.4
....etc.
end case

我认为这可以在MYSQL中完成，并且我可以如何理解它？

Answer 1

这可以通过连接同一个表的两个别名来完成，其中一个指向当前Record_ID，另一个指向前一个Record_ID，在连接条件中加1。 / p>

这只有在Record_ID的行保证顺序且没有间隙的情况下才有效。

SELECT
  pcur.Price,
  pcur.Price_Difference, 
  CASE 
    WHEN plast.Price < 50 THEN (pcur.Price_Difference / 0.05)
    WHEN plast.Price BETWEEN 50.001 AND 100 THEN (pcur.Price_Difference / 1)
    WHEN plast.Price BETWEEN 100.001 AND 150 THEN (pcur.Price_Difference / 1.25)
    WHEN plast.Price BETWEEN 150.001 AND 200 THEN (pcur.Price_Difference / 1.4)
    ELSE 999 # don't forget your else case
  END AS var_1
FROM
  /* First aliased table is the current record */
  prices pcur
  /* Second aliased table is the previous one, whose id is one behind */
  LEFT JOIN prices plast ON (pcur.Record_ID = plast.Record_ID + 1);

以下是演示：http://sqlfiddle.com/#!2/552ee/7

如果Record_ID序列中存在潜在的差距，则使用较慢的方法，使用子选择通过Price和ORDER BY从上一行检索LIMIT 1。这在大型数据集上没有超级性能。

SELECT 
  rid,
  Price,
  Price_Difference,
  CASE 
    WHEN var_1 < 50 THEN (Price_Difference / 0.05)
    WHEN var_1 BETWEEN 50.001 AND 100 THEN (Price_Difference / 1)
    WHEN var_1 BETWEEN 100.001 AND 150 THEN (Price_Difference / 1.25)
    WHEN var_1 BETWEEN 150.001 AND 200 THEN (Price_Difference / 1.4)
    ELSE 999 # don't forget your else case
  END AS var_1
FROM (
  SELECT
    Record_ID rid,
    Price, 
    Price_Difference,
   /* Subselect returns only one Price from the previous Record_ID which may not be in sequence */
   (SELECT Price FROM prices WHERE Record_ID < rid ORDER BY Record_ID DESC LIMIT 1) AS var_1
  FROM prices
) pcalc

以下是这个演示：http://sqlfiddle.com/#!2/552ee/5

MYSQL根据Case语句和其他行值计算Column值

1 个答案: