I've got multiple entries in table A and would like to display the number of entries in a coloumn of table B. Is there a way to create a dynamic cell-content displaying the number of entries in a table?
I'm a beginner in MySQL and did not find a way to do it so far.
Example table A:
+----+------+------------+
| id | name | birthday |
+----+------+------------+
| 1 | john | 1976-11-18 |
| 2 | bill | 1983-12-21 |
| 3 | abby | 1991-03-11 |
| 4 | lynn | 1969-08-02 |
| 5 | jake | 1989-07-29 |
+----+------+------------+
What I'd like in table B:
+----+------+----------+
| id | name | numusers |
| 1 | tblA | 5 |
+----+------+----------+
In my actual database there is no incrementing ID so just taking the last value would not work - if this would've been a solution.
If MySQL can't handle this the option would be to create some kind of cronjob on my server reading the number of rows and writing them into that cell. I know how to do this - just checking if there's another way.
I'm not looking for a command to run on the mysql-console. What I'm trying to figure out is if there's some option which dynamically changes the cell's value to what I've described above.
You can create a view that will give you this information. The SQL for this view is inspired by an answer to a similar question:
CREATE VIEW table_counts AS
SELECT table_name, table_rows
FROM information_schema.tables
WHERE table_schema = '{your_db}';
The view will have the cells you speak of. As you can see, it is just a filter on an already existing table, so you might consider that this table information_schema.tables is the answer to your question.
You can do that directly with COUNT() for example SELECT COUNT(*) FROM TblA The you get all rows from that table. If you IDXs are ok then its very fast. If you write it to another table you have to make an request too to get the result of the second table. So i think your can do it directly.
If you have some performance problems there are some other possibilities like Triggers or Stored Procedures to calculate that result and save them in a memory table to get a better performance.
Related
In short; we are trying to return certain results from one table based on second level criteria of another table.
I have a number of source data tables,
So:
Table DataA:
data_id | columns | stuff....
-----------------------------
1 | here | etc.
2 | here | poop
3 | here | etc.
Table DataB:
data_id | columnz | various....
-----------------------------
1 | there | you
2 | there | get
3 | there | the
4 | there | idea.
Table DataC:
data_id | column_s | others....
-----------------------------
1 | where | you
2 | where | get
3 | where | the
4 | where | idea.
Table DataD: etc. There are more and more will be added ongoing
And a relational table of visits, where there are "visits" to some of these other data rows in these other tables above.
Each of the above tables holds very different sets of data.
The way this is currently structured is like this:
Visits Table:
visit_id | reference | ref_id | visit_data | columns | notes
-------------------------------------------------------------
1 | DataC | 2 | some data | etc. | so this is a reference
| | | | | to a visit to row id
| | | | | 2 on table DataC
2 | DataC | 3 | some data | etc. | ...
3 | DataB | 4 | more data | etc. | so this is a reference
| | | | | to a visit to row id
| | | | | 4 on table DataB
4 | DataA | 1 | more data | etc. | etc. etc.
5 | DataA | 2 | more data | etc. | you get the idea
Now we currently list the visits by various user given criteria, such as visit date.
however the user can also choose which tables (ie data types) they want to view, so a user has to tick a box to show they want data from DataA table, and DataC table but not DataB, for example.
The SQL we currently have works like this; the column list in the IN conditional is dynamically generated from user choices:
SELECT visit_id,columns, visit_data, notes
FROM visits
WHERE visit_date < :maxDate AND visits.reference IN ('DataA','DataC')
The Issue:
Now, we need to go a step beyond this and list the visits by a sub-criteria of one of the "Data" tables,
So for example, DataA table has a reference to something else, so now the client wants to list all visits to numerous reference types, and IF the type is DataA then to only count the visits if the data in that table fits a value.
For example:
List all visits to DataB and all visits to DataA where DataA.stuff = poop
The way we currently work this is a secondary SQL on the results of the first visit listing, exampled above. This works but is always returning the full table of DataA when we only want to return a subset of DataA but we can't be exclusive about it outside of DataA.
We can't use LEFT JOIN because that doesn't trim the results as needed, we can't use exclusionary joins (RIGHT / INNER) because that then removes anything from DataC or any other table,
We can't find a way to add queries to the WHERE because again, that would loose any data from any other table that is not DataA.
What we kind of need is a JOIN within an IF/CASE clause.
Pseudo SQL:
SELECT visit_id,columns, visit_data, notes
FROM visits
IF(visits.reference = 'DataA')
INNER JOIN DataA ON visits.ref_id = DataA.id AND DataA.stuff = 'poop'
ENDIF
WHERE visit_date < 2020-12-06 AND visits.reference IN ('DataA','DataC')
All criteria in the WHERE clause are set by the user, none are static (This includes the DataA.stuff criteria too).
So with the above example the output would be:
visit_id | reference | ref_id | visit_data | columns | notes
-------------------------------------------------------------
1 | DataC | 2 | some data | etc. |
2 | DataC | 3 | some data | etc. |
5 | DataA | 1 | more data | etc. |
We can't use Union because the different Data tables contain lots of different details.
Questions:
There may be a very straightforward answer to this but I can't see it,
How can we approach trying to achieve this sort of partial exclusivity?
I suspect that our overarching architecture structure here could be improved (the system complexity has grown organically over a number of years). If so, what could be a better way of building this?
What we kind of need is a JOIN within an IF/CASE clause.
Well, you should know that's not possible in SQL.
Think of this analogy to function calls in a conventional programming language. You're essentially asking for something like:
What we need is a function call that calls a different function depending on the value you pass as a parameter.
As if you could do this:
call $somefunction(argument);
And which $somefunction you call would be determined by the function called, depending on the value of argument. This doesn't make any sense in any programming language.
It is similar in SQL — the tables and columns are fixed at the time the query is parsed. Rows of data are not read until the query is executed. Therefore one can't change the tables depending on the rows executed.
The simplest answer would be that you must run more than one query:
SELECT visit_id,columns, visit_data, notes
FROM visits
INNER JOIN DataA ON visits.ref_id = DataA.id AND DataA.stuff = 'poop'
WHERE visit_date < 2020-12-06 AND visits.reference = 'DataA';
SELECT visit_id,columns, visit_data, notes
FROM visits
WHERE visit_date < 2020-12-06 AND visits.reference = 'DataC';
Not every task must be done in one SQL query. If it's too complex or difficult to combine two tasks into one query, then leave them separate and write code in the client application to combine the results.
I have a situation in MS Access database that I must prevent duplicate records based on combination of three attributes:
StudentNumber
ColleagueID
TypeOfAttending
So, for one combination (StudentNumber & ColleagueID) I have three types of attending: A, B and C.
Here is an example:
+---------------+-------------+---------------+
| StudentNumber | ColleagueID | AttendingType |
+---------------+-------------+---------------+
| 100 | 10 | A |
| 100 | 10 | B |
| 100 | 10 | C |
| 100 | 11 | A |
| 100 | 11 | B |
| 100 | 11 | C |
| 100 | 11 | C |
+---------------+-------------+---------------+
So last row would not be acceptable.
Does anyone have any idea?
As noted, you could choose all 3 as a PK. Or you can even create a unique index on all 3 columns. These two ideas are thus code free.
Last but least, you could use a Before change macro,and do a search (lookup) in the table to check if the existing record exists. So far, given your information, likely a unique index is the least effort, and does not require you to change the PK to all 3 columns (which as noted is a another solution).
So, you could consider a before change macro. And use this:
Lookup a Record in MyTable
Where Condition = [z].[Field1]=[MyTable].[Field1] And
[z].[Field2]=[MyTable].[Field2] And
[z].[ID]<>[MyTable].[ID]
Alias Z
RaiseError -123
Error Description: There are other rows with this data
So, you can use a data macro, use the before change table macro. Make sure you have the raise error code indented "inside" of the look up code. And note how we use a alias for the look up, since the table name (MyTable) is already in context, and is already the current row of data, so we lookup using "z" as a alias to distinguish between the current row, and that of lookup record.
So, from a learning point of view, the above table macro can be used, but it likely less work and effort to simply setup a uniquie index on all 3 columns.
This question already has answers here:
Return rows in the exact order they were inserted
(4 answers)
Closed 4 years ago.
I don't know whether it is already answered. I hadn't got any answers.In Mysql tables, the rows will be arranged in the order of primary key. For example
+----+--------+
| id | name |
+----+--------+
| 1 | john |
| 2 | Bryan |
| 3 | Princy |
| 5 | Danny |
+----+--------+
If I insert anothe row insert into demo_table values(4,"Michael").The table will be like
+----+---------+
| id | name |
+----+---------+
| 1 | john |
| 2 | Bryan |
| 3 | Princy |
| 4 | Michael |
| 5 | Danny |
+----+---------+
But I need the table to be like
+----+---------+
| id | name |
+----+---------+
| 1 | john |
| 2 | Bryan |
| 3 | Prince |
| 5 | Danny |
| 4 | Michael |
+----+---------+
I want the row to be concatenated to the table i.e.,
The rows of the table should be in the order of insertion.Can anybody suggest me the query to get it.Thank you for any answer in advance.
There is in general no internal order to the records in a MySQL table. The only order which exists is the one you impose at the time you query. You typically impose that order using an ORDER BY clause. But there is a bigger design problem here. If you want to order the records by the time when they were inserted, then you should either add a dedicated column to your table which contains a timestamp, or perhaps make the id column auto increment.
If you want to go with the latter option, here is how you would do that:
ALTER TABLE demo_table MODIFY COLUMN id INT auto_increment;
Then, do your insertions like this:
INSERT INTO demo_table (name) VALUES ('Michael');
The database will choose an id value for the Michael record, and in general it would be greater than any already existing id value. If you need absolute control, then adding a timestamp column might make more sense.
Just add another Column Created (Timestamp) in your table to store the time of insertion
Then use this Command for insertion
insert into demo_table id, name,created values(4,"Michael",NOW())
The NOW() function returns the current date and time.
Since you are recording the timestamp, it can be also used for future reference too
It's not clear why you want to control the "order" in which the data is stored in your table. The relational model does not support this; unless you specify an order by clause, the order in which records are returned is not deterministic.. Even if it looks like data is stored in a particular sequence, the underlying database engine can change its mind at any point in time without breaking the standards or documented behaviours.
The fact you observe a particular order when executing a select query without order by is a side effect. Side effects are usually harmless, right up to the point where the mean feature changes and the side effect's behaviour changes too.
What's more - it's generally a bad idea to rely on the primary key to have "meaning". I assume your id column represents a primary key; you should really not rely on any business meaning in primary keys - this is why most people use surrogate keys. Depending on the keys indicating in which order a record was created is probably harmless, but it still seems like a side effect to me. In this, I don't support #TimBiegeleisen's otherwise excellent answer.
If you care about the order in which records were entered, make this explicit in the schema by adding a timestamp column, and write your select statement to order by that timestamp. This is the least sensitive to bugs or changes in the underlying logic/database engine.
I have a database which contains a lot of data and although I was not involved in setting it up it is what I have to work with.
Within this database is somewhat of a lookup table. However, this table has no link to any other tables. It essentially takes the following form
ID | input | table_name |
-------------------------------------
1 | Movie | movie_tbl |
2 | Cartoon | cartoon_tbl |
3 | Animation | cartoon_tbl |
4 | Audio | audio_tbl |
5 | Picture | picture_tbl |
The table is a lot larger than the above, but the structure is as above. So what happens is someone visits my site. Here, they have an input field. Say they enter Movie then the above table is called to find the input with Movie. It then gets what table it needs to look in. I would imagine that the query would be something like
SELECT table_name FROM lookup_table WHERE input LIKE Movie;
Now that should return movie_tbl. I now know that I need to search for Movie within movie_tbl and return all the data for its row. So movie_tbl might be like this (data would be some type of data and the column names different)
ID | input | col_1 | col_2 | col_3 |
----------------------------------------------------
1 | Movie | data | data | data |
2 | Cartoon | data | data | data |
3 | Animation | data | data | data |
4 | Audio | data | data | data |
5 | Picture | data | data | data |
So now my query will be something like this
SELECT * FROM movie_tbl WHERE input LIKE Movie;
Now the tables have tens of thousands of lines of data. My real question is whether the above will be effecient or not? With the database I was given however, I do not see any other way I could do this (I cant touch the database). Is there anything I can do to make this more effecient?
Any advice appreciated
Thanks
Why are you checking for input in the 2nd table? You have already filtered the input from the first table:
SELECT table_name FROM lookup_table WHERE input LIKE Movie;
In this case you dont have to make 2 queries. Just the 2nd one should suffice. Or just having Movie data in the 2nd table and separate tables for Cartoon, Animation etc. Because then you wont be accessing the 'WHERE' clause, just:
SELECT * FROM movie_tbl;
2nd Suggestion: Use = instead of LIKE. No need for pattern matching if you know the exact input string.
When a row is duplicated in our system, it inserts the new row with a reference back to the ID of the one it was duplicated from.
If that new row is then duplicated, it has a reference back to the row it was duplicated from and so on.
What I cant work out is how to follow this trail in a SELECT.
Take the following data...
+------+-------------------+
| ID | duplicated_from |
+------+-------------------+
| 1 | NULL |
| 2 | 1 |
| 3 | 2 |
| 4 | NULL |
+------+-------------------+
So, given ID 1, how would you look up all the slides in the chain that have been duplicated off it?
Or is this something that will have to be done at an application level?
Seems that you are after a recursive query, i found this solutions that may help you How to create a MySQL hierarchical recursive query
HTH