I have a Table called Payments, and a couple others called Invoices, Subs, Licenses.
Is it possible to add a reference to these tables in Payments? Something that would generate this
result => CREATE TABLE Payments (
Invoices table,
Subs table
)
I'm thinking in the same way I can put an object into an array in other languages.
EDIT: Im using MySQL. Basically, I want to map them to Java with one class called Payments and was wondering if I could do this on the db serverside. Otherwise I would have to figure out how to map all of them in the Javaclass. Which Im thinking now might be easier
In MySQL, you can have a subquery virtually anywhere you can use an expression. But it must return a single value.
You can use a "derived table", which is a subquery, as a table-like thing after FROM or JOIN.
What you typed may be better handled with a VIEW:
CREATE VIEW Results AS
SELECT i.*, s.*
FROM Invoices AS i
JOIN Subs AS s ON i.foo = s.foo
(where foo is the column used for relating those two tables together.)
Related
I'm currently thinking about a database schema in MySQL where I store SELECT queries into a certain table column, just to execute them on-the-fly when getting selected, and having the result passed instead of the actual query.
Would this be possible somehow? Or may this be bad practice? Is it even technically possible to have a result table passed to a single field, at least so I could run the query through PDO to get back a nested result array? Are there any alternatives?
I've read that this may be achieved through stored procedures, and although I grip the concept of those I can't think of how I could use those to achieve that.
You could do this, but what purpose do you have for doing it?
I would suggest using views:
The syntax should be valid when the view is created, unlike storing
the SQL in a field which may have invalid syntax.
It's easier to debug and modify.
For example, let's say one of the queries you want to store is:
SELECT product_category, COUNT(*) AS category_count
FROM product
GROUP BY product_category;
You can create a new "view" object that defines this query:
CREATE VIEW prod_cat_count AS
SELECT product_category, COUNT(*) AS category_count
FROM product
GROUP BY product_category;
Now, the object called "prod_cat_count" is stored in the database. Internally, the database just knows that "prod_cat_count" is equal to the SELECT query we mentioned. When the view is created, the database validates the syntax (checks that all columns exist, checks you haven't forgotten the GROUP BY, for example)
Then, whenever you want to get this data/run this query, you can run this statement (in SQL or in application code, for example):
SELECT product_category, category_count
FROM prod_cat_count;
If you then decide you want to change the way the product categories are counted, you can adjust the view:
SELECT product_category, COUNT(*) AS category_count
FROM product
GROUP BY product_category
ORDER BY product_category;
Hope that helps!
Here is a View called viewwithcommonfield :
SELECT
`schematopologytest01`.`talpha`.`CommonField` AS `CommonField_tAlpha`,
`schematopologytest01`.`tbeta`.`CommonField` AS `CommonField_tBeta`
FROM
(`schematopologytest01`.`talpha`
JOIN `schematopologytest01`.`tbeta`)
When I execute
SHOW FULL fields FROM viewwithcommonfield IN SchemaTopologyTest01
I get this:
How do I map the fields back to specific tables? Can I write a view against the tables in information_schema?
Here are the table structures that are referenced in the view. The tables share a common field called CommonField:
No, there is no metadata available to map views of a column back to the original column in a base table. That would require multiple tables, because any given expression in the select-list may reference multiple columns from different tables.
Consider:
SELECT CONCAT(
`schematopologytest01`.`talpha`.`AlphaFieldA`,
`schematopologytest01`.`tbeta`.`BetaFieldE`) AS `ConcatenatedField`
FROM `schematopologytest01`.`talpha`
JOIN `schematopologytest01`.`tbeta` ON ...
Which table and column would ConcatenatedField list as its origin? It would have to be stored in two rows of another INFORMATION_SCHEMA table.
There are also select-list expressions possible in a view that don't reference any base table:
CREATE VIEW ViewNow AS SELECT NOW() AS `now`;
What about columns that are themselves scalar subqueries? Or references to stored functions? Or an aggregate function like COUNT() or SUM() where the value is not found in any base table?
Many views do not derive their data from base tables deterministically at all. Edit: What I mean is that it's not always possible to know which rows or columns are the source of data in a view, because they results are combined in some ways. It's probably more clear to say that reversing the query to get the original data is not always possible, depending on the query.
It's not possible to update those views. But if there were metadata about where the data "came from," there would have to be something in the metadata to indicate that. It would be impractical because it would be complex, and have little value.
I have a one model car and table cars. In the table cars I have a lot of filed that are required to be there.
Is there any way to split this table by the filed so f1,f2,f3 go in table 'cars' and f6, f7, f8 in 'cars2'.
My goal is to at the end not to write two query for the two tables. Is this possible with eloquent.
Apparently I would not recommend the idea of splitting the table.
Because cars and cars2 would make the DB a redundant one , thereby causing a lot of ambiguity for coders to come.
However, I would recommend the use of join queries to gracefully access all the data needed from the 2 tables that your planning to use , using only a single query.
I am trying to use the results of another query to use as a criteria for another. In my specific example, I might have four houses that are 'A', 'B', 'C', 'D' (the unique values of a field in a table called Homes).
I want to go through another query and say for each house type, what percent of residents (in Residents table) are married, which I want to do by using Count() to count the number for each Home type.
Do I need to loop through the results using VBA? Asking on a higher level, is there a way to use the results from a query as inputs into another - more than just limit the results of the new query to the results of the prior query?
Edit:
In semi-pseudo code:
For each (result of previous query) Do
New query WHERE field1 = (row of previous query)
End Do
What I am trying to ask, is there a way to accomplish this in Access using SQL? Or is this something that has to be done in VBA?
I know that if it can be done in SQL that would be the best performing and best practice, but I'm relatively inexperienced in SQL and online resources aren't always helpful because Access has it's own particular flavor of SQL.
Since you are using VBA to run this, you can loop through your recordsets and yes you can use a value from one query in the next query. There are alot of resources out there to help.
VBA: Working with RecordSets
Looping through Record Sets
Code through all records
To answer your general question, yes there is. You can do a nested query i.e. select column a from table a where column a = (select column b from table b where column b=x)
You can go as many levels deep as you want, but the caveat is the nested query can only return one column and with a specific answer set. You can also use select statements as your columns i.e
select (select column b from table b) col b from table a ..... Not the exact syntax but I would have to dig out some examples from an old project to find that.
Nested queries are useful, but for the level of precision you are looking for, a stored procedure or a view is probably a better option. Just for ease of use, I would look at creating a view of the data that you want and then querying from that to start with. More flexible than a nested query.
You need to join two tables using a common column and then get your specific column from any of the table
SELECT A.REQUIRED_FIELD from TABLEA AS A
INNER JOIN TABLEB AS B ON A.FOREIGN_KEY=B.FOREIGN_KEY
WHERE CONDITION
I want to try and keep this as one query and not use PHP, but it's proving to be tough.
I have a table called applications, that stores all the applications and some basic information about them.
Then, I have a table with all the types of applications in it, and that table contains a reference to another table which stores more specific data about the specific type of application in question.
select applications.id as appid, applications.category, type.title as type, type.id as tid, type.valuefld, type.tablename
from applications
left join type on applications.typeid=type.id
left join department on type.deptid=department.id
where not isnull(work_cat)
and work_cat != ''
and applications.deleted=0
and datei between '10-04-14' and '11-04-14'
order by type, work_cat
Now, in the old version, there is another query on every single result. Over hundreds of results... that sucks.
This is the query I'd like to integrate so I can get all the data in one result row. (Old is ASP, I'm re-writing it in PHP)
query = "select sum("&adors.fields("valuefld")&") as cost, description from "&adors.fields("tablename")&" where appid = '"&adors.fields("tablename")&"'"
Prepared statements, I'm aware, are the best solution, but for now they are not an option.
You can't do this with a plain SQL query - you need to have a defined set of tables that your query is based on. The fact that your current implementation queries from whatever table is named by tablename from the first result-set means that to get this all in one query, you will have to restructure your data. You have to know what tables you're querying from rather than having it dynamic.
If the reason for these different tables is the different information stored in each requiring different record (column) structures, you might want to look into Key/Value pair storage in a large table. Once you combine the dynamically named ones into a single location you can integrate your two queries together.