Suppose we have 20 columns in a table and I want to return 19 of them.
How can I do that ?
select *
will give me all of them but I want only 19.
Is there a good solution for that situation ? something like
select * - [columnName]
?!?
Nope, sorry. You can take *, or you can take them one at a time, but you can't take "all of them except for X, Y, or Z."
As has been said, you use SELECT * for all columns or list the columns individually if you don't want them all.
Listing columns does seem like a chore but there is an important reason why it's actually good.
While it's OK for ad hoc queries, it's highly recommended that use don't use SELECT * in code because when the database schema changes you will get different columns in the results returned to your application which is almost certainly not what you want. If you could do select * but address from customer this would have the same problem: changing the DB would change the structure of the results of your query which is bad.
So not only can you not do it, I would recommend not doing it even if you could.
You can explicitly name each column you wish to select. That is the only way to exclude columns.
Related
Thank you for checking my question out!
I'm trying to write a query for a very specific problem we're having at my workplace and I can't seem to get my head around it.
Short version: I need to be able to target columns by their name, and more specifically by a part of their name that will be consistent throughout all the columns I need to combine or compare.
More details:
We have (for example), 5 different surveys. They have many questions each, but SOME of the questions are part of the same metric, and we need to create a generic field that keeps it. There's more background to the "why" of that, but it's pretty important for us at this point.
We were able to kind of solve this with either COALESCE() or CASE statements but the challenge is that, as more surveys/survey versions continue to grow, our vendor inevitably generates new columns for each survey and its questions.
Take this example, which is what we do currently and works well enough:
CASE
WHEN SURVEY_NAME = 'Service1' THEN SERV1_REC
WHEN SURVEY_NAME = 'Notice1' THEN FNOL1_REC
WHEN SURVEY_NAME = 'Status1' THEN STAT1_REC
WHEN SURVEY_NAME = 'Sales1' THEN SALE1_REC
WHEN SURVEY_NAME = 'Transfer1' THEN Null
ELSE Null
END REC
And also this alternative which works well:
COALESCE(SERV1_REC, FNOL1_REC, STAT1_REC, SALE1_REC) as REC
But as I mentioned, eventually we will have a "SALE2_REC" for example, and we'll need them BOTH on this same statement. I want to create something where having to come into the SQL and make changes isn't needed. Given that the columns will ALWAYS be named "something#_REC" for this specific metric, is there any way to achieve something like:
COALESCE(all columns named LIKE '%_REC') as REC
Bonus! Related, might be another way around this same problem:
Would there also be a way to achieve this?
SELECT (columns named LIKE '%_REC') FROM ...
Thank you very much in advance for all your time and attention.
-Kendall
Table and column information in Db2 are managed in the system catalog. The relevant views are SYSCAT.TABLES and SYSCAT.COLUMNS. You could write:
select colname, tabname from syscat.tables
where colname like some_expression
and syscat.tabname='MYTABLE
Note that the LIKE predicate supports expressions based on a variable or the result of a scalar function. So you could match it against some dynamic input.
Have you considered storing the more complicated properties in JSON or XML values? Db2 supports both and you can query those values with regular SQL statements.
Is there a way to determine if a trigger on any given table affects one other specific table?
My actual problems is rows being deleted from a table, and I am confident it happens from a trigger on another table. I need to find this "other table" (there are several hundred tabels).
Thx in advance!
Regards /Snedker
You can use
SELECT def,
t.*
FROM sys.triggers t
CROSS APPLY (SELECT OBJECT_DEFINITION(object_id)) C(def)
WHERE def LIKE '%DELETE%' AND def LIKE '%your_table%'
If that doesn't find anything and you want to broaden the search you can use
SELECT *
FROM sys.sql_modules
WHERE definition LIKE '%DELETE%' AND definition LIKE '%your_table%'
If that still doesn't work and you think that the query might be being sent by an application you can use extended events to get to the bottom of things as per my answer here.
I'm building an abstract gem. i need a sql query that looks like this
SELECT * FROM my_table WHERE * LIKE '%my_search%'
is that possible?
edit:
I don't care about querys performance because it's a feature function of a admin panel, which is used once a month. I also don't know what columns the table has because it's so abstract. Sure i could use some rails ActiveRecord functions to find all the columns but i hoped to avoid adding this logic and just using the *. It's going to be a gem, and i can't know what db is going to be used with it. Maybe there is a sexy rails function that helps me out here.
As I understand the question, basically you are trying to build a sql statement which should check for a condition across all columns in that table. A dirty hack, but this generates the required Sql.
condition_string = MyTable.column_names.join(' LIKE ? OR ')
MyTable.all(:conditions => [condition_string, '%my_search%'])
However, this is not tested. This might work.
* LIKE '...' isn't valid according to the SQL standards, and not supported by any RDBMS I'm aware of. You could try using a function like CONCAT to make the left argument of LIKE, though performance won't be good. As for SELECT *, it's generally something to be avoided.
No, SQL does not support that syntax.
To search all columns you need to use procedures or dynamic SQL. Here's another SO question which may help:
SQL: search for a string in every varchar column in a database
EDIT: Sorry, the question I linked to is looking for a field name, not the data, but it might help you write some dynamically SQL to build the query you need.
You didn't say which database you are using, as there might be a vendor specific solution.
Its only an Idea, but i think it worth testing!
It depends on your DB you can get all Columns of a table, in MSSQL for example you can use somethink like:
select name from syscolumns where id=object_id('Tablename')
Under Oracle guess its like:
select column_name from USER_TAB_COLUMNS where TABLE_NAME = 'Tablename'
and then you will have to go through these columns usign a procedure and maby a cursor so you can check for each Column if the data your searching for is in there:
if ((select count(*) from Tablename where Colname = 'searchingdata') > 0)
then keep the results in a separated table(ColnameWhereFound, RecNrWhereFound).
The matter of Datatye may be an Issue if you try to compare strings with numbers, but if you notice for instance under SQL-Server the syscolumns table contains a column called "usertype" which contains a number seems to refer to the Datatype stored in the Columne, like 2 means string and 7 means int, and 2 means smallint, guess Oracle would have something similar too.
Hope this helps.
I am wondering how others would handle a scenario like such:
Say I have multiple choices for a user to choose from.
Like, Color, Size, Make, Model, etc.
What is the best solution or practice for handling the build of your query for this scneario?
so if they select 6 of the 8 possible colors, 4 of the possible 7 makes, and 8 of the 12 possible brands?
You could do dynamic OR statements or dynamic IN Statements, but I am trying to figure out if there is a better solution for handling this "WHERE" criteria type logic?
EDIT:
I am getting some really good feedback (thanks everyone)...one other thing to note is that some of the selections could even be like (40 of the selections out of the possible 46) so kind of large. Thanks again!
Thanks,
S
What I would suggest doing is creating a function that takes in a delimited list of makeIds, colorIds, etc. This is probably going to be an int (or whatever your key is). And splits them into a table for you.
Your SP will take in a list of makes, colors, etc as you've said above.
YourSP '1,4,7,11', '1,6,7', '6'....
Inside your SP you'll call your splitting function, which will return a table-
SELECT * FROM
Cars C
JOIN YourFunction(#models) YF ON YF.Id = C.ModelId
JOIN YourFunction(#colors) YF2 ON YF2.Id = C.ColorId
Then, if they select nothing they get nothing. If they select everything, they'll get everything.
What is the best solution or practice for handling the build of your query for this scenario?
Dynamic SQL.
A single parameter represents two states - NULL/non-existent, or having a value. Two more means squaring the number of parameters to get the number of total possibilities: 2 yields 4, 3 yields 9, etc. A single, non-dynamic query can contain all the possibilities but will perform horribly between the use of:
ORs
overall non-sargability
and inability to reuse the query plan
...when compared to a dynamic SQL query that constructs the query out of only the absolutely necessary parts.
The query plan is cached in SQL Server 2005+, if you use the sp_executesql command - it is not if you only use EXEC.
I highly recommend reading The Curse and Blessing of Dynamic SQL.
For something this complex, you may want a session table that you update when the user selects their criteria. Then you can join the session table to your items table.
This solution may not scale well to thousands of users, so be careful.
If you want to create dynamic SQL it won't matter if you use the OR approach or the IN approach. SQL Server will process the statements the same way (maybe with little variation in some situations.)
You may also consider using temp tables for this scenario. You can insert the selections for each criteria into temp tables (e.g., #tmpColor, #tmpSize, #tmpMake, etc.). Then you can create a non-dynamic SELECT statement. Something like the following may work:
SELECT <column list>
FROM MyTable
WHERE MyTable.ColorID in (SELECT ColorID FROM #tmpColor)
OR MyTable.SizeID in (SELECT SizeID FROM #tmpSize)
OR MyTable.MakeID in (SELECT MakeID FROM #tmpMake)
The dynamic OR/IN and the temp table solutions work fine if each condition is independent of the other conditions. In other words, if you need to select rows where ((Color is Red and Size is Medium) or (Color is Green and Size is Large)) you'll need to try other solutions.
I am doing a simple SELECT statement in an Oracle DB and need to select the columns in a somewhat-specific order. Example:
Table A has 100 attributes, one of which is "chapter" that occurs somewhere in the order of columns in the table. I need to select the data with "chapter" first and the remaining columns after in no particular order. Essentially, my statement needs to read something like:
SELECT a.chapter, a. *the remaining columns* FROM A
Furthermore, I cannot simply type:
SELECT a.chapter, a.*
because this will select "chapter" twice.
I know the SQL statement seems simple, but if I know how to solve this problem, I can extrapolate this thought into more complicated areas. Also, let's assume that I can't just scroll over to find the "chapter" column and drag it to the beginning.
Thanks.
You should not select * in a program. As your schema evolves it will bring in things you do not know yet. Think about what happens when someone add a column with the whole book in it? The query you thought would be very cheap suddenly starts to bring in megabytes of data.
That means you have to specify every column you need.
Your best bet is just to select each column explicitly.
A quickie way to get around this would be SELECT a.chapter AS chapterCol, a.* FROM table a; This means there will be one column name chapterCol (assuming there's not a column already there named chapterCol. ;))
If your going to embed the 'SELECT *' into program code, then I would strongly recommend against doing that. As noted by the previous authors, your setting up the code to break if a column is ever added to (or removed from) the table. The simple advice is don't do it.
If your using this in development tools (viewing the data, and the like). Then, I'd recommend creating a view with the specific column order you need. Capture the output from 'SELECT COLUMN_NAME FROM ALL_TAB_COLUMNS' and create a select statement for the view with the column order you need.
This is how I would build your query without having to type all the names in, but with some manual effort.
Start with "Select a.chapter"
Now perform another select on your data base as follows :
select ','|| column_name
from user_tab_cols
where table_name = your_real_table_name
and column_name <> 'CHAPTER';
now take the output from that, in a cut-and-paste manner and append it to what you started with. Now run that query. It should be what you asked for.
Ta-da!
Unless you have a very good reason to do so, you should not use SELECT * in queries. It will break your application every time the schema changes.