I have two identical tables (apart from the name) and when trying to run the below, I get Error Code: 1054. Unknown column 'pages_jj.pageid' in 'where clause'
SELECT pages.pageid,pages.maintext FROM databasename.pages
WHERE pages.pageid=pages_jj.pageid LIMIT 50000;
Please help!
You have to add the pages_jj table to the FROM clause, like:
SELECT pages.pageid,pages.maintext FROM databasename.pages, databasename.pages_jj
WHERE pages.pageid=pages_jj.pageid LIMIT 50000;
You need to mention the two tables in your FROM clause, otherwise it will not know what pages_jj is.
SELECT pages.pageid,pages.maintext FROM databasename.pages, databasename.pages_jj
WHERE pages.pageid=pages_jj.pageid LIMIT 50000;
But be warned that this will return a cross product of the two tables. If pageid values are unique in table pages_jj , then no problem. However, if pages_jj can contain multiple lines with the same pageid, then lines from pages will be selected as many times as there are lines with the same pagesid in pages_jj... which may or may not be what you are expecting.
To get only one occurrence per matching line in pages, use the following:
SELECT pages.pageid, pages.maintext FROM databasename.pages
WHERE EXISTS (SELECT pageid FROM databasename.pages_jj WHERE pages.pageid=pages_jj.pageid) LIMIT 50000;
Related
My database is called: (training_session)
I try to print out some information from my data, but I do not want to have any duplicates. I do get it somehow, may someone tell me what I do wrong?
SELECT DISTINCT athlete_id AND duration FROM training_session
SELECT DISTINCT athlete_id, duration FROM training_session
It works perfectly if i use only one column, but when I add another. it does not work.
I think you misunderstood the use of DISTINCT.
There is big difference between using DISTINCT and GROUP BY.
Both have some sort of goal, but they have different purpose.
You use DISTINCT if you want to show a series of columns and never repeat. That means you dont care about calculations or group function aggregates. DISTINCT will show different RESULTS if you keep adding more columns in your SELECT (if the table has many columns)
You use GROUP BY if you want to show "distinctively" on a certain selected columns and you use group function to calculate the data related to it. Therefore you use GROUP BY if you want to use group functions.
Please check group functions you can use in this link.
https://dev.mysql.com/doc/refman/8.0/en/group-by-functions.html
EDIT 1:
It seems like you are trying to get the "latest" of a certain athlete, I'll assume the current scenario if there is no ID.
Here is my alternate solution:
SELECT a.athlete_id ,
( SELECT b.duration
FROM training_session as b
WHERE b.athlete_id = a.athlete_id -- connect
ORDER BY [latest column to sort] DESC
LIMIT 1
) last_duration
FROM training_session as a
GROUP BY a.athlete_id
ORDER BY a.athlete_id
This syntax is called IN-SELECT subquery. With the help of LIMIT 1, it shows the topmost record. In-select subquery must have 1 record to return or else it shows error.
MySQL's DISTINCT clause is used to filter out duplicate recordsets.
If your query was SELECT DISTINCT athlete_id FROM training_session then your output would be:
athlete_id
----------
1
2
3
4
5
6
As soon as you add another column to your query (in your example, the column called duration) then each record resulting from your query are unique, hence the results you're getting. In other words the query is working correctly.
UPDATE:
In the following question, I thought when you select rows, MySQL create a kind of row index, and then the LIMIT and OFFSET clause cuts off this list by its numbers. The problem is I'm using wrong values or field combination to sort the rows as the have almost the same value.
Here is my mistake, assuming to much "intelligence" form MySQL ;-)
The solution is always to have a more reliable sorting field as a fallback, like the ID, like so ORDER BY priority DESC, id ASC
This is a strict MySQL question. Why does it seem the LIMIT OFFSET clause is applied before the ORDER BY? or what am I missing?
Here is the example, first we select a list of rows ordered by a field called priority:
SELECT d0_.name, d0_.id AS id_0, d0_.priority AS priority_1 FROM destination d0_ WHERE d0_.active = 1 ORDER BY d0_.priority DESC;
The results looks like this:
Then I want to select the first 10 rows from this list using the following query:
SELECT d0_.name, d0_.id AS id_0, d0_.priority AS priority_1 FROM destination d0_ WHERE d0_.active = 1 ORDER BY d0_.priority DESC LIMIT 10 OFFSET 10;
And I've got this result:
Why does not the list goes from "Grandvalira" to "Sierra nevada"?
The problem of this, is not the actual order but some rows are never reached! like "Vallnord Ordino-Arcalís". As I change the OFFSET value, it does not go through all the rows, and it event repeats some rows.
This is the basic question. But this is giving me problems at the end when using the "KnpPaginatorBundle (2.5.3)" of Symfony. I thought was a problem of the php code, but mysql queries are giving this unexpected results for me.
Any help or clue of whats going on?
Thanks!
You are not getting the results you are expecting because your data has many rows with the same value for priority.
When you use 'order by' on priority, all the rows with priority can come in any order. There is no guarantee about the ordering with the same value of priority. To resolve the tie, you can add additional fields to your order by clause. Depending on your choice you might choose to include name or id field in the 'order by' clause.
return multiple data
why?
mysql sql :
select * from t_book where id=(select round(max(id)*rand()) from t_book)
As suggested by #Tarek you might have duplicated id values. To find out which you can run this query:
SELECT id, COUNT(*) c FROM t_book GROUP BY id HAVING c > 1;
On the assumption that "id" is a unique primary key value, it shouldnt return multiple values. In the returned values do all of the "id" fields match, or is it returning multiple varying ids?
You have a Syntax error,
The function Round takes the form:
ROUND(N,[D]);
The problem is not with your id column's uniqueness.
select round(max(id)*rand()) from t_book
This will return various number of ids, at least in mysql 5.6. I don't know why, but it's really weird.
You can try this, for selecting a random record from your table:
select * from t_book order by rand() limit 1;
If you stick with this round-max-random method, keep in mind that round can return 0 too and it's unlikely that you have a 0 id.
First I thought this is caused by duplicate id values. You can try this fiddle, and see what happens: http://sqlfiddle.com/#!9/7fc510/1.
For multiple runs I got 0, 1 or 2 result records.
When we are retrieving data from database, we use something like this
SELECT * FROM Names
However, we will get all the data inside the specific table. Since I am going to update some data to the database and want to make a comparison bewteen the last row of data in the db and the most updated data, how can I select and retrieve the last two row of the database only?
If you were using SQL Server, you would do something like this:
SELECT TOP 2 * FROM Names ORDER BY Name DESC
SQL Server syntax:
SELECT TOP 2 column_name FROM table_name ORDER BY column_name DESC;
Example:
If you want to retrieve the last value of the customer_name column from the customers table, you need to write the following query:
SELECT LAST (CUSTOMER_NAME) AS LAST_CUSTOMER FROM CUSTOMERS;
You should include a column in the Names table to keep track of when a name was added to the table since you cannot guarantee that the rows are in the order that they were inserted. With that column you can use the order by clause..
In MySQL Syntax:
SELECT *
FROM Names
ORDER BY order_column DESC
LIMIT 2;
If you want to get the last rows as they are, you cannot order the table by the inserted names because that is just getting the 2 names that come last in an alphabetically sorted list of names. You could try something like this where you include an offset in the limit clause if you get the number of rows in the table:
SELECT *
FROM Names
LIMIT *num_rows*-2, 2;
You would have to know the number of rows to use this query or use an additional query to implement a row count that works with limit. This, however, still may not be accurate since the system may not order the rows as they were inserted. I still recommend the first option where you keep track of order/time a name was inserted.
I have the following SQL query , it seems to run ok , but i am concerned as my site grows it may not perform as expected ,I would like some feeback as to how effective and efficient this query really is:
select * from articles where category_id=XX AND city_id=XXX GROUP BY user_id ORDER BY created_date DESC LIMIT 10;
Basically what i am trying to achieve - is to get the newest articles by created_date limited to 10 , articles must only be selected if the following criteria are met :
City ID must equal the given value
Category ID must equal the given value
Only one article per user must be returned
Articles must be sorted by date and only the top 10 latest articles must be returned
You've got a GROUP BY clause which only contains one column, but you are pulling all the columns there are without aggregating them. Do you realise that the values returned for the columns not specified in GROUP BY and not aggregated are not guaranteed?
You are also referencing such a column in the ORDER BY clause. Since the values of that column aren't guaranteed, you have no guarantee what rows are going to be returned with subsequent invocations of this script even in the absence of changes to the underlying table.
So, I would at least change the ORDER BY clause to something like this:
ORDER BY MAX(created_date)
or this:
ORDER BY MIN(created_date)
some potential improvements (for best query performance):
make sure you have an index on all columns you querynote: check if you really need an index on all columns because this has a negative performance when the BD has to build the index. -> for more details take a look here: http://dev.mysql.com/doc/refman/5.1/en/optimization-indexes.html
SELECT * would select all columns of the table. SELECT only the ones you really require...