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.
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.
I'm still a bit of a noob, so pardon if this question is a bit obvious. I did search for an answer but either couldn't understand how the answers I found applied, or simply couldn't find an answer.
I have a massive database housed on a DB2 for i server which I'm accessing using SQL through SQLExplorer (based on Squirrel SQL). The tables are very poorly documented and the first order of business is figuring out how to find my way around.
I want to write a simple query that does this:
1) Allows me to search the entire database looking for tables that include a column called "Remarks" (which contains field descriptions).
2) I then want it to search that column for a keyword.
3) I want a table returned that includes the names of the tables that include that keyword (just the name, I can look up the table alphabetically later and look inside if I need to.)
I need this search to be super lightweight, and I'm hoping the concept I describe will achieve that. Anything that eats up a lot of resources will likely anger the sys admin for the server.
Just to show I have tried (and that I am a complete noob), here's what I've got so far.
SELECT *
FROM <dbname>
WHERE Remarks LIKE '<keyword>'
Feel free to mock, I told you I'm an idiot :-).
Any help? Perhaps at least a push in the right direction?
PS - I can't seem to find a search function in SQLExplorer, if someone knows if I can perhaps use a simple search or filter to accomplish this same goal...that would be great.
You can query the system catalog to identify the tables:
SELECT TABLE_SCHEMA, TABLE_NAME, COLUMN_NAME
FROM QSYS2.SYSCOLUMNS WHERE UPPER(DBILFL) = 'REMARKS'
And then query each table individually:
SELECT * FROM TABLE_SCHEMA.TABLE_NAME WHERE Remarks LIKE '%<keyword>%'
See the LIKE predicate for details of the pattern expression.
Normally i use something like this
SELECT TABLE_SCHEMA, TABLE_NAME
,COLUMN_NAME,SYSTEM_COLUMN_NAME,COLUMN_HEADING
,DATA_TYPE, "LENGTH",NUMERIC_SCALE
FROM QSYS2.SYSCOLUMNS
WHERE UPPER(COLUMN_NAME) LIKE '%REMARK%'
#JamesA, i'm at V6R1, by default, normal user are not authorized to object QADBIFLD in QSYS
Generally, many if not most IBM i shops (especially those that use RPG) stick to 10 (or less) character schema names & table names, and have a 10 (or less) character names for 'system' column names, even if longer column names are also provided. Column text generally describes each field.
SELECT SYSTEM_TABLE_SCHEMA, SYSTEM_TABLE_NAME
,SYSTEM_COLUMN_NAME,
,DATA_TYPE, "LENGTH",NUMERIC_SCALE
,CHAR(COLUMN_TEXT)
FROM QSYS2.SYSCOLUMNS
WHERE UPPER(COLUMN_NAME) LIKE '%REMARK%'
Apologies for the somewhat confusing Title, I've been struggling to find an answer to my question, partly because it's hard to concisely describe it in the title line or come up with a good search string for it. Anyhoooo, here's the problem I'm facing:
Short version of the question is:
How can I write the following (invalid but understandable SQL) in valid SQL understood by Oracle:
select B.REPLACER as COL, A.* except A.COL from A join B on a.COL = B.COL;
Here's the long version (if you already know what I want from reading the short version, you don't need to read this :P ):
My (simplified) task is to come up with service that massages a table's data and provide it as a sub-query. The table has a lot of columns (a few dozens or more), and I am stuck with using "select *" rather than explicitly listing out all columns one by one, because new columns may be added to or removed from the table without me knowing, although my downstream systems will know and adjust accordingly.
Say, this table (let's call it Table A from now on) has a column called "COL", and we need to replace the values in that COL with the value in the REPLACER column of table B where the two COL value matches.
How do I do this? I cannot rename the column because the downstream systems expect "COL"; I cannot do without the "expect A.COL" part because that would cause the sql to be ambiguous.
Appreciate your help, almighty StackOverflow
Ray
You can either use table.* or table.fieldName.
There is no syntax available for table.* (except field X).
This means that you can only get what you want by explicitly listing all of the fields...
select
A.field1,
A.field2,
B.field3,
A.field4,
etc
from
A join B on a.COL = B.COL;
This means that you may need to re-model your data so as to ensure you don't keep getting new fields. OR write dynamic sql. Interrogate the database to find out the column names, use code to write a query as above, and then run that dynamically generated query.
Try this: (not tested)
select Case B.COL
when null then A.COL
else B.REPLACER
end as COLAB, A.*
from A left join B on A.COL = B.COL;
This should get the B.REPLACER when exists B.COL = A.COL, you can add more column in the select (like sample col1, col2) or use A.* (change COL into COLAB to make it distinguish with A.COL in A.*) .
Like said before, you cannot specify in regular sql which column not to select. you could write a procedure for that, but it would be quite complex, because you would need to return a variable table type. Probably something with refcursor magic stuff.
The closest I could come up with is joining with using. This will give you the column col in the first field once and for the rest all columns in a and b. So not what you want basically. :)
select *
from a
join b using (col)
Let's start from first principles. select * from .... is a bug waiting to happend and has no place in production code. Of course everybody uses it because it entails less typing but that doesn't make it a good practice.
Beyond that, the ANSI SQL standard doesn't support select * except col1 from .... syntax. I know a lot of people wish it would but it doesn't.
There are a couple of ways to avoid excessive typing. One is to generate the query by selecting from data dictionary, using one of the views like USER_TAB_COLUMNS. It is worth writing the PL?SQL block to do this if you need lots of queries like this.
A simpler hack is to use the SQL*Plus describe to list out the structure of table A; cut'n'paste it into an IDE which supports regular expressions and edit the columns to give you the query's projection.
Both these options might strike you as labourious but frankly either workaround (and especially the second) would have taken less effort than asking StackOverflow. You'll know better next time.
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.
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.