Let's say I record my car's latitude & longitude every minute or so, adding a row to a table each time.
I want to have a DB grid with 4 columns
latitude
longitude
distance since last measurment
curent street address, if known
Number 4, I can try to retrieve from Google Maps and I either get a text or blank, so let's ignore that.
How do I get #3? Should I calculate it in my application or in MySql (using a stored procedure or just a complicated SELECT)?
Whichever answer, can someone provide some sample code, or a link to an example? Google is not my friend today :-(
You can do it either way:
Calculate it in your query, and display it in a calculated column (such as when showing line totals where you're displaying ITEM_PRICE, QUANTITY, ITEM_PRICE * QUANTITY AS LINE_COST.
Calculate it in your application, using a calculated field (double-click the TTable component to open the Fields Editor, add field, set the type to ftCalculated, and write code in the OnCalcEvent for that calculated field.
The first choice is usually preferable, because the DB is usually more efficient at doing those sorts of thing on sets of data than when your code does it by row instead. (Set operations are obviously more efficient than row operations.)
Ken’s answer is goes far as it goes, because the most difficult task of solving your problem is getting data from two rows at the same time in SQL. If you were using Oracle or SQL Server, you could use the analytical/windowing LAG function. With MySQL you have to do it yourself. Here are 2 articles on how to simulate LAG in MySQL
http://www.onlamp.com/pub/a/mysql/2007/03/29/emulating-analytic-aka-ranking-functions-with-mysql.html?page=1
http://explainextended.com/2009/03/12/analytic-functions-optimizing-lag-lead-first_value-last_value/
Plus there are StackOverflow questions on how to get data from the previous row.
How do I lag columns in MySQL?
Calculate delta(difference of current and previous row) in sql
I cannot offer any opinion on any of them because I seldom work with MySQL.
Related
I have a table that contains data from repeated experiments (for example, site A has one sample, and the lab processed the sample three times obtaining slightly different values). I need to average these results in a separate table, but what I have read on the Microsoft support site is that a query that pulls data into another table with a calculated field is not possible on Access.
Can I query multiple data points from one table into a single calculated field in another table? Thank you.
UPDATE
I ended up doing a lot of manual adjustments of the file format to create a calculated field in the existing table that averages each sites data, so my problem is, for my current purposes, solved. However I would still like to understand. Following up with you both, I think the problem was that I had repeated non-unique IDs between rows when I probably should have made data columns with unique variable names so that I could query each variable name for an average.
So, instead of putting each site separately on the y axis, I formatted it by putting the sample number for each site on the x-axis:
I was able to at least create a calculated field using this second format in order to create an average value for each site.
Would have there been a way to write a query using the first method? Luckily, my data set was not at all very hefty, so I could handle a reformat manually, but if the case were with thousands of data entries, I couldn't have done that.
Also, here is the link to the site I mentioned originally https://support.office.com/en-ie/article/add-a-calculated-field-to-a-table-14a60733-2580-48c2-b402-6de54fafbde3.
Thanks all.
I have a table traffic with 7 columns, namely toll_id, date1, shift, car_single, car_return, car_local and car_total.
How could I populate first 5 columns manually, and then store a value in column car_total, which will be the sum of car_single and car_return?
Here is the image of my table:
Just to add a 3rd and 4th ways of achieving the desired outcome:
If you have at least MySQL v5.7.6, you can use a generated column as car_total.
Alternatively, you can choose not to store car_total at all, but calculate this value on the fly while querying the table.
Having a column to store the results of the calculation is good if you regularly have search based on that field because you can use indexes to speed up the searches. Calculating the results on the fly may be better, if you just need to display the result of the calculation, but there is no need to filter on it.
There are two ways to do this:
Add the logic in the application itself, so that it calculates total before inserting the record. (Recommended)
Write an after insert trigger (http://dev.mysql.com/doc/refman/5.7/en/trigger-syntax.html) which calculates the count when record is inserted.
I have job in Talend that is designed to bring together some data from different databases: one is a MySQL database and the other a MSSQL database.
What I want to do is match a selection of loan numbers from the MySQL database (about 82,000 loan numbers) to the corresponding information we have housed in the MSSQL database.
However, the tables in MSSQL to which I am joining the data from MySQL are much larger (~ 2 million rows), are quite wide, and thus cost much more time to query. Ideally I could perform an inner join between the two tables based on the loan number, but since they are in different databases this is not possible. The inner join that is performed inside a tMap occurs after the Lookup input has already returned its data set, which is quite large (especially since this particular MSSQL query will execute a user-defined function for each loan number).
Is there any way to create a global variable out of the output from the MySQL query (namely, the loan numbers selected by the MySQL query) and use that global variable as an IN clause in the MSSQL query?
This should be possible. I'm not working in MySQL but I have something roughly equivalent here that I think you should be able to adapt to your needs.
I've never actually answered a Stackoverflow question and while I was typing this the page started telling me I need at least 10 reputation to post more than 2 pictures/links here and I think I need 4 pics, so I'm just going to write it out in words here and post the whole thing complete with illustrations on my blog in case you need more info (quite likely, I should think!)
As you can see, I've got some data coming out of the table and getting filtered by tFilterRow_1 to only show the rows I'm interested in.
The next step is to limit it to just the field I want to use in the variable. I've used tMap_3 rather than a tFilterColumns because the field I'm using is a string and I wanted to be able to concatenate single quotes around it but if you're using an integer you might not need to do that. And of course if you have a lot of repetition you might also want to get a tUniqueRows in there as well to save a lot of unnecessary repetition
The next step is the one that does the magic. I've got a list like this:
'A1'
'A2'
'B1'
'B2'
etc, and I want to turn it into 'A1','A2','B1','B2' so I can slot it into my where clause. For this, I've used tAggregateRow_1, selecting "list" as the aggregate function to use.
Next up, we want to take this list and put it into a context variable (I've already created the context variable in the metadata - you know how to do that, right?). Use another tMap component, feeding into a tContextLoad widget. tContextLoad always has two columns in its schema, so map the output of the tAggregateRows to the "value" column and enter the name of the variable in the "key". In this example, my context variable is called MyList
Now your list is loaded as a text string and stored in the context variable ready for retrieval. So open up a new input and embed the variable in the sql code like this
"SELECT distinct MY_COLUMN
from MY_SECOND_TABLE where the_selected_row in ("+
context.MyList+")"
It should be as easy as that, and when I whipped it up it worked first time, but let me know if you have any trouble and I'll see what I can do.
I started building a search engine monitor. I'm pulling data from the google rest api into a mysql database with the following fields: date, search-keyword, domain, url, position.
Now I got into trouble querying and outputting the data for charting. The results go up and down, new results from google come into the list which haven't been there on the first day. However for charting I have to assign the first days at least blank values to output a chart.
What I do right now: First I select every domain showing up in the period. Lets say the for the keyword searchengine I get the domains wikipedia.org, ixquick.com, yahoo.com, searchenginewatch.com When I make another request for ever domain to query an array of rankings grouped by day. leading to the ...
Problem: Is where any query (mysql/nosql) which returns for each day an average and if where is no row a default value e.g. blank?
Result should look like:
dates={01/01/2014,02,03,04,05,06,07,08,...,31}
wikipedie={1,1,1,1,1,1,1,1,...,1}
yahoo = {"","",7,5,3,3,3,...,3}
You can create a date table, select the date range you'd like, and outer join your data to it, filling in 0s for values that do not exist for a given term/date.
Edit:
Some more details.
1) Create a table that has a row for every date +- 10 years (or whatever is appropriate). You can make this one column if you'd like, or many columns (date, month, year, etc.). The second approach makes this extensible if you want to summarize by various rollups in the future.
2) Outer join your table to the date table and use a NVL statement to coerce any null averages to 0.
3) Profit!
If your results are grouped by date, how can MySQL know there's (for example) 31 days in that month?
On the other hand, you can somehow fill the holes in PHP by loop through the array and fill a zero if the value does not exist.
I'm trying to help a taxi company. The problem is that they have a credit card machine that takes payments, and has its own database entries, and there is a completely separate database that has a list of entries such as pick up time, and drop off time.
I need to match the database of trip entries to the credit card purchases, and the only way to do this is by matching which vehicle is running the transaction, and looking for a time CLOSE TO the DROP OFF time and see if it's a match. It's not ideal.
So, I am trying to compare two times in yy-mm-dd hh:mm:ss format. They need to be plus or minus 5 minutes of each other. How do I do this in MYSQL?
I thought SubTime and Addtime would work, and it seemed to, but then I got wierd results.
SELECT * FROM completedtrans WHERE DrivID = 128 AND TransTime BETWEEN SUBTIME('2013-06-20 16:53:06', '0 00:05:00') AND ADDTIME('2013-06-20 16:53:06', '0 00:05:00')
Here's an example of one of my searches. Can anyone tell me what's wrong with it? It's supposed to search 5 minutes before and after that particular given time. I can't simply write the time, because the query is automatically generated through php code.