问题
| time | company | quote |
+---------------------+---------+-------+
| 0000-00-00 00:00:00 | GOOGLE | 40 |
| 2012-07-02 21:28:05 | GOOGLE | 60 |
| 2012-07-02 21:28:51 | SAP | 60 |
| 2012-07-02 21:29:05 | SAP | 20 |
How do I do a lag on this table in MySQL to print the difference in quotes, for example:
GOOGLE | 20
SAP | 40
回答1:
This is my favorite MySQL hack.
This is how you emulate the lag function:
SET @quot=-1;
select time,company,@quot lag_quote, @quot:=quote curr_quote
from stocks order by company,time;
lag_quote
holds the value of previous row's quote. For the first row @quot is -1.curr_quote
holds the value of current row's quote.
Notes:
order by
clause is important here just like it is in a regular window function.- You might also want to use lag for
company
just to be sure that you are computing difference in quotes of the samecompany
. - You can also implement row counters in the same way
@cnt:=@cnt+1
The nice thing about this scheme is that is computationally very lean compared to some other approaches like using aggregate functions, stored procedures or processing data in application server.
EDIT:
Now coming to your question of getting result in the format you mentioned:
SET @quot=0,@latest=0,company='';
select B.* from (
select A.time,A.change,IF(@comp<>A.company,1,0) as LATEST,@comp:=A.company as company from (
select time,company,quote-@quot as change, @quot:=quote curr_quote
from stocks order by company,time) A
order by company,time desc) B where B.LATEST=1;
The nesting is not co-related so not as bad (computationally) as it looks (syntactically) :)
Let me know if you need any help with this.
回答2:
From MySQL 8.0 and above there is no need to simulate LAG
. It is natively supported,
Window Function :
Returns the value of expr from the row that lags (precedes) the current row by N rows within its partition. If there is no such row, the return value is default. For example, if N is 3, the return value is default for the first two rows. If N or default are missing, the defaults are 1 and NULL, respectively.
SELECT
company,
quote,
LAG(quote) OVER(PARTITION BY company ORDER BY time) AS prev_quote
FROM tab;
DBFiddle Demo
回答3:
To achieve the desired result, first you need to find the last and next to last timestamps for each company. It is quite simple with the following query:
SELECT c.company, c.mts, max(l.ts) AS lts
FROM (SELECT company, max(ts) AS mts FROM cq GROUP BY company) AS c
LEFT JOIN cq l
ON c.company = l.company AND c.mts > l.ts
GROUP BY c.company, c.mts;
Now you have to join this subquery with the original table to get the desired results:
SELECT c.company, l.quote, coalesce(l1.quote, 0),
(l.quote - coalesce(l1.quote, 0)) AS result
FROM (SELECT c.company, c.mts, max(l.ts) AS lts
FROM (SELECT company, max(ts) AS mts FROM cq GROUP BY company) AS c
LEFT JOIN cq l
ON c.company = l.company AND c.mts > l.ts
GROUP BY c.company, c.mts) AS c
LEFT JOIN cq AS l ON l.company = c.company AND l.ts = c.mts
LEFT JOIN cq AS l1 ON l1.company = c.company AND l1.ts = c.lts;
You can observe results on SQL Fiddle.
This query is using only standard SQL capabilities and should work on any RDBMS.
来源:https://stackoverflow.com/questions/11303532/simulate-lag-function-in-mysql