How can I write this mysql query without using functions?
SELECT word
FROM dvddict
JOIN dvdrevdict
ON word=RIGHT(revword, 4)
WHERE LENGTH(revword)=9;
The dvddict table has a column word and the dvdrevdict table has a column revword, which is just the reverse of the word column. I want to write it without having to use both RIGHT() and LENGTH() functions.
Thanks!!
Add two columns revword_right and revword_length to dvdrevdict, then have a trigger populate them on insert or update.
Unfortunately MySQL has no function based indexes which could probably help you (in the where length.. case) as well if your goal is to speed things up.
Related
I have to explore undocumented schemas to come up with query statements that will satisfy some business need.
When I SELECT * FROM foo JOIN bar ON foo.barid=bar.id; I get back a list of columns and I could guess where the columns from one table end and the next begin. But it would be awfully convenient if it just used the columns' full names i the output. i.e, every column would display as foo.columnname or bar.columnname.
yes, that's bulkier than optimal, and no I'd never use it in a production solution. but for exploratory pokings and prodings it would make things easier when I'm trying to figure out why a query isn't working right.
How do I turn that on by default?
CLARIFICATION: No. I'm not looking for "how to list all columns in a table/schema. I want to run queries joining tables together, and see the results, and see unambiguously and easily, what table a given field came from.
You can query the database information_schema to help you figure out what is what in your database. Running the following will get you close:
select table_name, column_name from information_schema.columns
https://dev.mysql.com/doc/refman/8.0/en/information-schema-columns-table.html
If you must use * you can qualify by table for example
select foo.*,'//',bar.*
will display all columns from foo first then a divider then all columns from bar and within foo the display left to right represents the ordinal position of the columns in foo. If the display columns are left justified then the column datatype is string of some description (varchar,char,text etc) if right justified then a number of some sort (int,decimal,float etc) . If a number is left justified then the underlying datatype is string. Date datatypes in mysql are in the form yyyy-mm-dd so if you see this then the underlying dataype is likely to be date. Similarly datetime
To understand the actual datatypes and find the indexes,constraints and foreign keys on a table then show create table tablename . If you want all table definitions use workbench export or mysqldump utility.
Also do read up on what information_schema https://dev.mysql.com/doc/refman/8.0/en/information-schema.html can do for you and consider reverse engineering your DB in workbench.
I'm building a MySQL event to make a copy of a table in the database with a timestamp in the name.
CREATE TABLE `db_name`.`tbl_prefix_(SELECT TO_SECONDS(NOW()))` ( [the rest...]
Obviously this isn't working. What should I do to make it work?
Any suggestions are welcome.
Thanks
This is a bad architecture. Generating tables on the fly is not something you should do.
Instead, create a single table with a timestamp column. For instance, if you would before have 3 tables with three timestamps A, B, and C, you now have one table with a timestamp column containing the values A, B, and C, respectively.
In order to do this, you would need to use "dynamic SQL". That is, make use of the MySQL PREPARE statement.
What you'd need to do is populate a variable with a string that contains the SQL text you want to execute. Doing variable substitution into string is trivial.
The "trick" is to take that dynamic string and execute it like it was a SQL statement.
And that's what the PREPARE statement does for us, takes in a variable, and reads the contents of that variable like it were a SQL statement.
With that said, rather than give an example code that demonstrates this in more detail, I'm going to suggest that you re-think this idea of creating a table with timestamp value as part of the name.
What problem is that designed to solve? And carefully consider whether the proposed design for a solution will introduce a bigger problem than it solves.
I have a very large number of rows in my table, table_1. Sometimes I just need to retrieve a particular row.
I assume, when I use SELECT query with WHERE clause, it loops through the very first row until it matches my requirement.
Is there any way to make the query jump to a particular row and then start from that row?
Example:
Suppose there are 50,000,000 rows and the id which I want to search for is 53750. What I need is: the search can start from 50000 so that it can save time for searching 49999 rows.
I don't know the exact term since I am not expert of SQL!
You need to create an index : http://dev.mysql.com/doc/refman/5.1/en/create-index.html
ALTER TABLE_1 ADD UNIQUE INDEX (ID);
The way I understand it, you want to select a row with id 53750. If you have a field named id you could do this:
SELECT * FROM table_1 WHERE id = 53750
Along with indexing the id field. That's the fastest way to do so. As far as I know.
ALTER table_1 ADD UNIQUE INDEX (<collumn>)
Would be a great first step if it has not been generated automatically. You can also use:
EXPLAIN <your query here>
To see which kind of query works best in this case. Note that if you want to change the where statement (anywhere in the future) but see a returning value in there it will be a good idea to put an index on that aswell.
Create an index on the column you want to do the SELECT on:
CREATE INDEX index_1 ON table_1 (id);
Then, select the row just like you would before.
But also, please read up on databases, database design and optimization. Your question is full of false assumptions. Don't just copy and paste our answers verbatim. Get educated!
There are several things to know about optimizing select queries like Range and Where clause Optimization, the documentation is pretty informative baout this issue, read the section: Optimizing SELECT Statements. Creating an index on the column you evaluate is very helpfull regarding performance too.
One possible solution You can create View then query from view. here is details of creating view and obtain data from view
http://www.w3schools.com/sql/sql_view.asp
now you just split that huge number of rows into many view (i. e row 1-10000 in one view then 10001-20000 another view )
then query from view.
I am pretty sure that any SQL database with a little respect for themselves does not start looping from the first row to get the desired row. But I am also not sure how they makes it work, so I can't give an exact answer.
You could check out what's in your WHERE-clause and how the table is indexed. Do you have a proper primary key? Like using a numeric data type for that. Do you have indexes on more columns, that is used in your queries?
There is also alot to concider when installing the database server, like where to put the data and log files, how much memory to give the server and setting the growth. There's a lot you can do to tune your server.
You could try and split your tables in partitions
More about alter tables to add partitions
Selecting from a specific partition
In your case you could create a partition on ID for every 50.000 rows and when you want to skip the first 50.000 you just select from partition 2. How to do this ies explained quite well in the MySQL documentation.
You may try simple as this one.
query = "SELECT * FROM tblname LIMIT 50000,0
i just tried it with phpmyadmin. WHERE the "50,000" is the starting row to look up.
EDIT :
But if i we're you i wouldn't use this one, because it will lapses the 1 - 49999 records to search.
I hava nest query:
SELECT PIXEL_X as 'X_Coord', PIXEL_Y as 'Y_Coord',
CONVERTWATTS2DBM_udf(SUM(L2_VALUE)/SUM(L3_VALUE)) as 'Pixel_Value'
FROM table
WHERE
('GSM 850/900' like CONCAT('%',FILTER2,'/%') OR
'GSM 850/900' like CONCAT('%/',FILTER2,'%') )
GROUP BY X_Coord, Y_Coord;
but takes a long time, could you help me to improve their performance?
Thanks
A straight forward method to optimize would be this:
Create the filter variable yourself, in whatever language you use to access the database.
Set one query for each option for "GMS 850/900", then join them together using UNION, like so:
SELECT PIXEL_X as 'X_Coord', PIXEL_Y as 'Y_Coord',
CONVERTWATTS2DBM_udf(SUM(L2_VALUE)/SUM(L3_VALUE)) as 'Pixel_Value'
FROM table WHERE
'GSM 850/900' like '%YOURVALUE1%'
UNION
SELECT PIXEL_X as 'X_Coord', PIXEL_Y as 'Y_Coord',
CONVERTWATTS2DBM_udf(SUM(L2_VALUE)/SUM(L3_VALUE)) as 'Pixel_Value'
FROM table WHERE
'GSM 850/900' like '%YOURVALUE2%'
This should speed up the query.
Furthermore, it would speed up the query alot if you generated the values you generate on the fly beforehand. You could create a column and generate the CONVERTWATTS2DBM_udf in a ON WRITE trigger. This would remove the necessity of running this function not only on every row, but also on every run of the query itself.
Lastly, a composite index over Pixel_X, Pixel_Y and your newly created column could speed up the query further.
I have column in table where i store tag ids as 1|5|10
I want to explode the column using mysql query
What you probably might want to do is not store your tag ids like this. Make a separate table for them and your problem will dissapear naturally.
That's called normalisation.
You can chain SUBSTRING_INDEX to accomplish an explode effect, but let me remind you this is a horrible solution over database normalisation, as suggested by Kos.
If you were trying to select all MySQL entries with a 5 in your middle column (#|5|#) you would use this query:
SELECT * FROM table WHERE SUBSTRING_INDEX(SUBSTRING_INDEX(table.field, '|', 2), '|', -1) = 5
#Everyone: this is a horrible solution so please don't PLAN to use it; however, some times we're in a situation where a quick one liner like this must be used until a finer inspection on the database schema can happen.