Problem with union the table which not exists - mysql

I want to get data from several tables which has the same column name in one SQL statement, for example:
SELECT name, age FROM table_a UNION SELECT name, age FROM table_b UNION...
But the table_x may not exists that I can't avoid from people who send the request to me, if one of the tables is not exits in a query it will be failed, is there any syntax to avoid that?
I know a way that I can use show tables to get all tables in the database and compare them to the request parameters first, but I hope I can do it from MySQL syntax.

The short answer is no. If you are using another language in front of it, such PHP or any other language really, you can query the tables as you suggest, but SQL expects the query to be accurate syntactically and if it's not it will error. There is one (IMO bad) way to do this, if you must. You could use a stored procedure, which would allow you to dynamically build the query as you would in PHP or another language, but that's about all you have with MySQL (or any database that I know).

Related

How to get the last inert id from TADOQuery?

When using mysql.h, there's a property called insert_id on the MYSQL connection object. Is there a similar feature when using a TADOConnection?
I am well ware of SELECT LAST_INSERT_ID();. What I'm asking is: If the C API has that feature, why is that feature missing in the ADO connector, if missing?
And if the only solution is to INSERT and SELECT the last insert id, are these two queries atomic, can some other INSERT queries be executed in-between, from a different connection, and make LAST_INSERT_ID return an unexpected value?
Since SQL-Server has OUTPUT Inserted.ID and Postgres has RETURNING id, I find MySQL to have made a dumb design choice with LAST_INSERT_ID().

Select SQL query FROM table, select using that query?

I'm trying to do the following in SQL alone. In the end it will end up on a wso2 DSS server but if it can be done in sql alone even better :)
Sudocode
Array results=Array;
result = <sql>select id, query from definitions</sql>
foreach result.query
r=<sql>query</sql>
results.push(r)
I am trying to run a select on table a that returns 2 columns.
One of the two columns is named query, and I want to then execute that query returning
id, query_title, query_text
We can assume that the query column always returns the same columns (through aliases written in the query)
The other option would be doing this in WSO2 DSS however I though at least that it can only do what sql does. Maybe joining with the ESB I could get it if that doesn't work but really my goal would be to do it ALL in sql as I am going to insert insert information and then update it into another table anyway.
You cannot do this with a single select query.
One solution is to do this in two steps. Fetch the query in the application using your SQL and then execute the second query from the application.
The second solution is to use a stored procedure and prepare/execute. How you then fetch the results depends on the nature of the results

How to use local DB table in a pass-through query?

I am currently working on a query in Access 2010 and I am trying to get the below query to work. I have the connection string between my local DB and the server that I am passing through to working just fine.
Select column1
, column2
from serverDB.dbo.table1
where column1 in (Select column1 from tbl_Name1)
In this situation table1 is the table on the server that I am passing through to get to, but the tbl_Name1 is the table that is actually in my Access DB that I am trying to use to create constraints on the data that I am pulling from the server.
When I try to run the query, I am getting the error that it doesn't think tbl_Name1 exists.
Any help is appreciated!
I just came across a solution that may help others in a similar situation.
This approach is easy because you can just run one query on your local Access database and get everything you need all at once. However, a lot of filtering/churning-through-results may be done on your own local computer behind the scenes, as opposed to on the remote server, so it may not necessarily be quick.
Steps
Create a query, make it a "Pass Through" query, and set up its "ODBC Connect Str" property to connect to the remote database.
Write the pass through query, something like SELECT RemoteId From RemoteTable and give your pass through query a name, maybe PassThroughQuery
Create a new query, make it a regular "Select" query.
Write your new query, using the pass through query you just created as a table in this new query (seems weird to use a query as a table, but it works) and join that PassThroughQuery "table" to your local table and filter it based on values in the local table, something like SELECT R.RemoteId, L.LocalValue FROM PassThroughQuery R INNER JOIN LocalTable L ON L.LocalId = R.RemoteId where L.LocalValue = 'SomeText'
This approach allows you to mix/join the results of a pass through query and the data in a local Access database table cleanly, albeit potentially slowly if there is a lot of data involved.
I think the issue is that a pass through query is one that is run on the server. Since one of the tables is located on the local Access file, it won't find the table.
Possible workaround if you must stay with the pass-through is you can build an SQL string with the results of the nested query rather than the query string itself (depending on the number of results this may or may not be practical)
e.g. Instead of Select column1 from tbl_Name1 you use "c1result1","c1result2",....

Is it possible to do a JOIN without knowing ON what column the JOIN is done?

Is there, for example, a function such as SELECT t0.*, t1.* JOIN t1 ON t0.some_known_field = GET_PRIMARY_INDEX(t1)?
No, it is not possible, in pure SQL, to use any sort of variable or lookup to construct the names of tables or columns.
You can use "dynamic SQL" which is a fancy-sounding way of having a stored procedure construct a text string from queries and other information and then run it as a SQL statement. Or you can do the same in your application program. Many SQL applications do this sort of thing.

Saving commands for later re-use in MySQL?

What would be the equivalant in MySQL for:
Saving a command for later reuse.
eg: alias command1='select count(*) from sometable;'
Where then I simply type command 1 to get the count for SomeTable.
Saving just a string, or rather part of a command.
eg: select * from sometable where $complex_where_logic$ order by attr1 desc;
WHere $complex_where_logic$ is something I wish to save and not have to keep writing out
Another approach is to create a view with your $complex_where_logic$ and query the view instead of the tables:
CREATE VIEW my_view AS SELECT * FROM sometable WHERE $complex_where_logic$
SELECT my_column FROM my_view ORDER BY some_column
Whenever you query a view, you always get the up-to-date data. Internally, MySQL runs the SELECT given in the CREATE VIEW statement and queries the results in order to obtain the results of your current SELECT. Therefore, a view does not improve performance compared to a single query. There a two main advantages in using views:
you have simpler SELECT statements since you do not have to type complex WHERE or JOIN Syntax again and again
you can use it to control user privileges, e.g. give a user access to a view but not to the original tables; this is not useful in your example, but - for example - you can think of views containing aggregate data only
This "template" feature could be part of client tools. Basically I use Toad. It has record macros feature. I think it is possible to do.
I take it the answer you are looking for isn't 'stored procedures'...?
I found that the best solution for this is just any rich GUI for SQL queries (TOAD, mysql query browser, etc). They offer the ability to save commands and browse them and well, of course, much more.