When I try to create a simple update query with Subtraction or anything else in the same table, it doesn't return me any value, could it be something to do with formating?
This is SQL code: UPDATE Sales SET Sales.GrossProfit = [SubTot]-[Cost];
It doesn't return any value.
I have copied syntax from different databases (from colleagues) where the same type of query worked. Could it be something to do with my general Access settings?
UPDATE Sales SET Sales.GrossProfit = [SubTot]-[Cost];
I expect the query to update the GrossProfit column with the calculation "SubTot - Cost"
Strictly speaking, an UPDATE query does not "return" any columns. It only changes data in existing rows, but by itself does not return those changes back to you. (It is more accurately called an Update statement since it does not return data.)
For instance, if you were to execute the UPDATE statement in VBA code, you would not get a RecordSet of columns like with a SELECT query. To get the results after executing the UPDATE statement would require that you execute a separate SELECT query.
However, I assume that you are viewing the Update query in the Access query designer. Correct?
In that case, when you click on the View button (Datasheet View), Access tries to be smart & useful about the UPDATE and converts it temporarily into a SELECT query so that you can visualize the existing data in the columns. The Datasheet View does not actually run the UPDATE statement, nor does it show a preview of what will happen when you do run it.
While in Design View or SQL View for an Update query (or for Delete, Make Table, and Append queries), there is a Run button with a red exclamation point (on the Design ribbon/toolbar). Clicking that button will actually execute the query that changes the table values. Although it will not show you results row by row, you should get a pop-up telling you how many rows were changed due to the statement. (You may also get confirmation prompts that the statement is about to change data in the table.)
Related
I am working with an application which needs to function with any of 300+ different MySQL databases on the same server. The databases all have nearly identical table structures, with slight variations. For example, a particular column might be present in a table for only some of the databases.
I'm wondering if there is a way that, when performing an update on a table, I can update a specific column if it exists, but still successfully execute if the column does not exist.
For example, say I have a basic update statement like this:
UPDATE some_table
SET col1 = "some value",
col2 = "another value",
col3 = "a third value"
WHERE id = 567
What can I do to make it so that, if col3 doesn't actually exist when that query is run, the statement still executes and col1 and col2 are still updated with the new values?
I have tried using IF and CASE, but those seem to only allow changing the value based on some condition, not whether or not a column actually gets updated.
I know I can query the database for the existence of the column, then use a simple if condition in the application code use a different query. However, that requires me to query the database twice: once to see if the column exists, and again to actually update it. I'd prefer to do it with one SQL query if possible. I feel like that application code might start to get unwieldy with lots of extra code to check the existence of this-or-that column and conditionally build queries, instead of just having one query which works regardless of which database the application happens to be running against at the time.
To clarify, any given instance of the application is ever only running against one database; there is a different application instance for each database, but the instances will all be running the same code. These are legacy databases that legacy code is also relying on, so I don't want to modify the actual structures in the database to make them more consistent, for fear of breaking the legacy code.
No, the syntax of your SQL query, including all column identifiers you reference, must be fixed at the time it is parsed, before it validates that the columns exist.
A given UPDATE will either succeed fully or fail fully. There is no way to update some of the columns if the query fails to update all of them.
You have two choices:
Query INFORMATION_SCHEMA.COLUMNS first, to check what columns exist in the table for a given schema. Then format your UPDATE query, including clauses to set each column only if the column exists in that instance of the table.
Or...
Run several UPDATE statements, one for each column you want to update. Each statement will succeed or fail independently, but you can catch the error and continue on to the remaining statements. You can put all these statements in a transaction, so the set of changes is committed atomically, regardless of how many succeed (a single failed statement does not roll back a transaction).
Either way, it requires you to write more code. That's the unavoidable cost of supporting such variable table structure.
what is the query that outputs the sql statement used to create a table?
You should check it in the output window of the mySQL workbench. It shows the query you executed along with many other useful information like the success or failure result of the query execution, time taken, affected row count etc. I've marked the last query I executed in red box.
Here is a chunk of the SQL I'm using for a Perl-based web application. I have a number of requests and each has a number of accessions, and each has a status. This chunk of code is there to update the table for every accession_analysis that shares all these fields for each accession in a request.
UPDATE accession_analysis
SET analysis_id = ? ,
reference_id = ? ,
status = ? ,
extra_parameters = ?
WHERE analysis_id = ?
AND reference_id = ?
AND status = ?
AND extra_parameters = ?
and accession_id is (
SELECT accesion_id
FROM accessions
where request_id = ?
)
I have changed the tables so that there's a status table for accession_analysis, so when I update, I update both accession_analysis and accession_analysis_status, which has status, status_text and the id of the accession_analysis, which is a not null auto_increment variable.
I have no strong idea about how to modify this code to allow this. My first pass grabbed all the accessions and looped through them, then filtered for all the fields, then updated. I didn't like that because I had many connections with short SQL commands, which I understood to be bad, but I can't help but think the only way to really do this is to go back to the loop in Perl holding two simpler SQL statements.
Is there a way to do this in SQL that, with my relative SQL inexperience, I'm just not seeing?
The answer depends on which DBMS you're using. The easiest way is to create a trigger on one table that provides the logic of updating the other table. (For any DB newbies -- a trigger is procedural code attached to a table at the DBMS (not application) layer that runs in response to an insert, update or delete on the table.). A similar, slightly less desirable method is to put the logic in a stored procedure and execute that instead of the update statement you're now using.
If the DBMS you're using doesn't support either of these mechanisms, then there isn't a good way to do what you're after while guaranteeing transactional integrity. However if the problem you're solving can tolerate a timing difference in the two tables' updates (i.e. The data in one of the tables is only used at predetermined times, like reporting or some type of batched operation) you could write to one table (live) and create a separate process that runs when needed (later) to update the second table using data from the first table. The correctness of allowing data to be updated at different times becomes a large and immovable design assumption, however.
If this is mostly about connection speed, then one option you have is to write a stored procedure that handles the "double update or insert" transparently. See the manual for stored procedures:
http://dev.mysql.com/doc/refman/5.5/en/create-procedure.html
Otherwise, You probably cannot do it in one statement, see the MySQL INSERT syntax:
http://dev.mysql.com/doc/refman/5.5/en/insert.html
The UPDATE syntax allows for multi-table updates (not in combination with INSERT, though):
http://dev.mysql.com/doc/refman/5.5/en/update.html
Each table needs its own INSERT / UPDATE in the query.
In fact, even if you create a view by JOINing multiple tables, when you INSERT into the view, you can only INSERT with fields belonging to one of the tables at a time.
The modifications made by the INSERT statement cannot affect more than one of the base tables referenced in the FROM clause of the view. For example, an INSERT into a multitable view must use a column_list that references only columns from one base table. For more information about updatable views, see CREATE VIEW.
Inserting data into multiple tables through an sql view (MySQL)
INSERT (SQL Server)
Same is true of UPDATE
The modifications made by the UPDATE statement cannot affect more than one of the base tables referenced in the FROM clause of the view. For more information on updatable views, see CREATE VIEW.
However, you can have multiple INSERTs or UPDATEs per query or stored procedure.
UPDATE sb_reviews
SET clients_id=(SELECT clients_id
FROM sb_users
WHERE id=sb_reviews.users_id)
sb_reviews is a table with a users_id and and a newly created clients_id. I'm trying to grab the clients_id from the sb_users table and put it into the sb_reviews table where sb_users.id=sb_reviews.users_id
This query does not work though, I get no records changed.
There is nothing wrong with your query.
In mysql, if the update has no effect (ie the value in the column isn't changing), it reports as not an update.
If you have already run this, you'll get "no records changed" message every run thereafter (unless data in the second table changes, of course).
I am trying to run an INSERT statement on table X each time I SELECT any record from table Y is there anyway that I can accomplish that using MySQL only?
Something like triggers?
Short answer is No. Triggers are triggered with INSERT, UPDATE or DELETE.
Possible solution for this. rather rare scenario:
First, write some stored procedures
that do the SELECTs you want on
table X.
Then, restrict all users to use only
these stored procedures and do not
allow them to directly use SELECT on table
X.
Then alter the stored procedures to
also call a stored procedure that
performs the action you want
(INSERT or whatever).
Nope - you can't trigger on SELECT - you'll have to create a stored procedure (or any other type of logging facility - like a log file or what ever) that you implicitly call on any query statement - easier if you create a wrapper that calls your query, calls the logging and returns query results.
If you're trying to use table X to log the order of SELECT queries on table Y (a fairly common query-logging setup), you can simply reverse the order of operations and run the INSERT query first, then run your SELECT query.
That way, you don't need to worry about linking the two statements with a TRIGGER: if your server crashes between the two statements then you already logged what you care about with your first statement, and whether the SELECT query runs or fails has no impact on the underlying database.
If you're not logging queries, perhaps you're trying to use table Y as a task queue -- the situation I was struggling with that lead me to this thread -- and you want whichever session queries Y first to lock all other sessions out of the rows returned so you can perform some operations on the results and insert the output into table X. In that case, simply add some logging capabilities to table Y.
For example, you could add an "owner" column to Y, then tack the WHERE part of your SELECT query onto an UPDATE statement, run it, and then modify your SELECT query to only show the results that were claimed by your UPDATE:
UPDATE Y SET owner = 'me' WHERE task = 'new' AND owner IS NULL;
SELECT foo FROM Y WHERE task = 'new' AND owner = 'me';
...do some work on foo, then...
INSERT INTO X (output) VALUES ('awesomeness');
Again, the key is to log first, then query.