In mysql, the creation statement of query table is as follows: show create table table01.
Now I have a requirement. I want to query the creation statement of a field in this table. What SQL can I use to meet my requirement?
I used Google search for a long time without finding a satisfactory answer, and MySQL official website did not provide the SQL statement I wanted.
I hope a simple SQL statement can make me get the following :
ALTER TABLE `table01`
ADD COLUMN `value` int(0) NULL DEFAULT NULL COMMENT 'value'
Second edit
Thank you for your help. I'm from China. My English is not very fluent. I'm sorry that I don't express myself accurately in some places.
In fact, the reason why I have this requirement is that the project I am currently working on is agile development. The iteration speed between versions is very fast. Often because of the change of requirements, the database tables and fields will increase with it. But I am lazy, and I don't want to record the fields every time I add them. So I use java to make an automation program. At present, it can be compared quickly Quickly find out the differences between formal and test database tables and fields.
But I wonder if there is a better way to find out the creation statement of this field, so that my program looks more perfect.
springboot-mysql-table-field-compare
Snowflakes fluttering north wind rustling(雪花飘飘北风萧萧)
Related
I took on a volunteer project a few years ago. The site is set up with Joomla, but most of the articles are rendered with php scripts that pull info from non-Joomla tables. The database is now almost 50MB and several of the non-Joomla tables have 60,000+ rows -- I had no idea it would get this big. Even just pulling up the list of the articles that contain these scripts takes a long time -- and right now there are only about 30 of them. I initially thought the problem was because I'm on dial-up, so everything is slow, but then we started getting "resources exceeded" notices, so I figured I better find out what's going on. It's not a high traffic site -- we get less than 2,000 unique visitors in any given month.
In one particular instance, I have one table where the library holdings (books, etc.) are listed by title, author, pub date, etc. The second table contains the names mentioned in those books. I have a Joomla! article for each publication that lists the names found in that book. I also have an article that lists all of the names from all of the books. That is the query below -- but even the ones for the specific books that pull up only 1,000 or so entries are very slow.
I originally set up indexes for these tables (MyISAM), but when I went back to check, they weren't there. So I thought re-configuring the indexes would solve the problem. Not even -- and according to EXPLAIN, they aren't even being used.
One of my problematic queries is as follows:
SELECT *
FROM pub_surnames
WHERE pub_surname_last REGEXP '^[A-B]'
ORDER BY pub_surname_last, pub_surname_first, pub_surname_middle
EXPLAIN gave:
id 1
select_type SIMPLE
table pub_surnames
type ALL
possible_keys NULL
key NULL
key_len NULL
ref NULL
rows 56422
Extra Using where; Using filesort
Also, phpmyadmin says "Current selection does not contain a unique column."
All of the fields are required for this query, but I read here that it would help if I listed them individually, so I did. The table contains a primary key, and I added a second unique index containing the primary key for the table, as well as the primary key for the table that holds the information about the publication itself. I also added an index for the ORDER BY fields. But I still get the same results when I use EXPLAIN and the performance isn't improved at all.
I set these tables up within the Joomla! database that the site uses for connection purposes and it makes it easier to back everything up. I'm wondering now if it would help if I used a separate database for our non-Joomla tables? Or would that just make it worse?
I'm not really sure where to go from here.
I think you are probably approaching this the wrong way. Probably it was the quick way to get it done when you first set it up, but now that the data has grown you are paying the price.
It sounds like you are recreating a massive list "inside" an article each time a page is rendered. Even though the source data is constantly being updated you would probably be better off storing the results. (Assuming I understand your data structure correctly.) Not knowing exactly what your php scripts are doing makes it a little complicated .. it could be that it would make more sense to actually make a very simple component to read the data from the other tables but I'll assume that doesn't make sense.
Here's what I think you might want to do.
Create a cron job (really easy to make a script using Joomla, go take a look at the jacs respository) and use it to run whatever your php is doing. You can schedule it once a day or once an hour or every 10 minutes, whatever makes sense.
Save the results. These could go into a data base table or you could cache them in the file system. Or both. Or possibly have the script update the articles since they seem to be fixed (you aren't adding new ones etc)
Then when the user comes you just want to either read the article if you stored there or you want to have a component that renders the results or make a plugin that will manage the queries for you. You should not be doing queries directly from inside an article layout, it's just wrong, even if no one knows it's there. If you have to run queries, use a content plugin similar to maybe the profile plugin, which does the queries in the right place architecturally.
Not knowing the exact purpose of what you are doing, it's hard to advise more, but I think if you are managing searches for people you'd likely be better off creating a way to use finder to index and search the results.
Check out below suggestions
Try changing your database engine to InnoDB which will work better for large datasets.
Also use RegEx alternative, which is used in "WHERE" part of query which hugely affects queries execution time.
Instead selecting all the columns with "*" just select needed columns.
Is it possible to create a custom function in MySQL like SUM, MAX, and so on. That accepts multiple columns and do some operation on each row?
The reason I am asking this question is because I tried to do my logic using stored procedure but unfortunatelly couldn't find a way how to select data from table name where the name of the table is input parameter.
Somebody suggested to use dynamic SQL but I can not get the cursor. So my only hope is to use custom defined function.
To make the question more clear here is what I want to do:
I want to calculate the distance of a route where each row in the database table represents coordinates (latitude and longtitude). Unfortunatelly the data I have is really big and if I query the data and do the calculationgs using Java it takes more than half a minute to transfer the data to the web server so I want to do the calculations on the SQL machine.
Select something1, something2 from table_name where table name is a variable
Multiple identically-structured tables (prerequisite for this sort of query) is contrary to the Principle of Orthogonal Design.
Don't do it. At least not without very good reason—with suitable indexes, (tens of) millions of records per table is easily enough for MySQL to handle without any need for partitioning; and even if one does need to partition the data, there are better ways than this manual kludge (which can give rise to ambiguous, potentially inconsistent data and lead to redundancy and complexity in your data manipulation code).
I have a large database w/ a bunch of tables and columns are mixed some allowing NULL while others not allowing NULL..
I just recently decided to STANDARDIZE my methods and USE NULL for all empty fields etc.. therefore i need to set ALL COLUMNS in ALL my tables to allow NULL (except for primaries ofcourse)
I can whip up a php code to loop this , but i was wondering if there's a quick way to do it via SQL?
regards
You can use meta data from system tables to determine your tables, columns, types etc. And then using that, dynamically build a string that contains your UPDATE SQL, with table and column names concatented in to it. This is then executed.
I've recently posted a solution that allowed the OP to search through columns looking for those that contain a particular value. In lieu of anyone providing a more complete answer, this should give you some clues about how to approach this (or at least, what to research). You'd need to either provide table names, or join to them, and then do something similar as this except you'd be checking type, not value (and the dynamic SQL you build would build an update, not a select).
I will be in a position to help you with your specific scenario further in a few hours... If by then you've had no luck with this (or other answers) then I'll provide something more complete then.
EDIT: Just realised you've tagged this as mySql... My solution was for MS SQL Server. The principals should be the same (and hence I'll leave this answer up as i think youll find it usefull), assuming MySql allows you to query its metadata, and execute dynamically generated SQL commands.
SQL Server - Select columns that meet certain conditions?
Okay, mysql indexing. Is indexing nothing more than having a unique ID for each row that will be used in the WHERE clause?
When indexing a table does the process add any information to the table? For instance, another column or value somewhere.
Does indexing happen on the fly when retrieving values or are values placed into the table much like an insert or update function?
Any more information to clearly explain mysql indexing would be appreciated. And please dont just place a link to the mysql documentation, it is confusing and it is always better to get a personal response from a professional.
Lastly, why is indexing different from telling mysql to look for values between two values. For Example: WHERE create_time >= 'AweekAgo'
I'm asking because one of my tables is 220,000+ rows and it takes more than a minute to return values with a very simple mysql select statement and I'm hoping indexing will speed this up.
Thanks in advanced.
You were down voted because you didn't make effort to read or search for what you are asking for. A simple search in google could have shown you the benefits and drawbacks of Database Index. Here is a related question on StackOverflow. I am sure there are numerous questions like that.
To simplify the jargons, it would be easier to locate books in a library if you arrange the in shelves numbered according to their area of specialization. You can easily tell somebody to go to a specific location and pick the book - that is what index does
Another example: imagine an alphabetically ordered admission list. If your name start with Z, you will just skip A to Y and get to Z - faster? If otherwise, you will have to search and search and may not even find it if you didn't look carefully
A database index is a data structure that improves the speed of operations in a table. Indexes can be created using one or more columns, providing the basis for both rapid random lookups and efficient ordering of access to records.
You can create an index like this way :
CREATE INDEX index_name
ON table_name ( column1, column2,...);
You might be working on a more complex database, so it's good to remember a few simple rules.
Indexes slow down inserts and updates, so you want to use them carefully on columns that are FREQUENTLY updated.
Indexes speed up where clauses and order by.
For further detail, you can read :
http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html
http://www.tutorialspoint.com/mysql/mysql-indexes.htm
There are a lot of indexing, for example a hash, a trie, a spatial index. It depends on the value. Most likely it's a hash and a binary search tree. Nothing really fancy because most likely the fancy thing is expensive.
I have a MySQL table about 1000 million records. It is very slow when I make a query.
So I split this table by ID into 10 sub-tables with the same structrue.
table_1(1-1000000)
table_2(10000001-2000000)
table_3(20000001-3000000)
......
But how can i query data in a fast way after table splitting?
when I query a user like this: select name from table where name='mark', I don't know go to which table for querying beacuse I can get the ID range.
Splitting tables this way is totally not the right way when you show your example query. You created more issues actually than solving anything.
Let's get back to the big table:
Step 1 is to see why it is slow, so post explain sql command to get an overview.
Step 2 is to see whether you can improve that query. Stating things like indexes are not a good solution can be true. If so please provide measurements showing this.
Step 3 is to think outside the box. You are running queries in a very big table which gets constantly inserts. Consider using a specifically for search designed index. For example consider indexing with Solr for the search commands.
Eventually you might even get to the hardware point, it just can't get faster on this hardware. But first follow through steps, add the right information, concrete measurements and specifications so you can get even more complete support on your case.