I'm using a PostgreSQL with a Go driver. Sometimes I need to query not existing fields, just to check - maybe something exists in a DB. Before querying I can't tell whether that field exists. Example:
where size=10 or length=10
By default I get an error column "length" does not exist, however, the size column could exist and I could get some results.
Is it possible to handle such cases to return what is possible?
EDIT:
Yes, I could get all the existing columns first. But the initial queries can be rather complex and not created by me directly, I can only modify them.
That means the query can be simple like the previous example and can be much more complex like this:
WHERE size=10 OR (length=10 AND n='example') OR (c BETWEEN 1 and 5 AND p='Mars')
If missing columns are length and c - does that mean I have to parse the SQL, split it by OR (or other operators), check every part of the query, then remove any part with missing columns - and in the end to generate a new SQL query?
Any easier way?
I would try to check within information schema first
"select column_name from INFORMATION_SCHEMA.COLUMNS where table_name ='table_name';"
And then based on result do query
Why don't you get a list of columns that are in the table first? Like this
select column_name
from information_schema.columns
where table_name = 'table_name' and (column_name = 'size' or column_name = 'length');
The result will be the columns that exist.
There is no way to do what you want, except for constructing an SQL string from the list of available columns, which can be got by querying information_schema.columns.
SQL statements are parsed before they are executed, and there is no conditional compilation or no short-circuiting, so you get an error if a non-existing column is referenced.
Related
I have to produce a dynamically generated T-SQL script that inserts records into various tables. I've done a bunch of searching and testing but can't seem to find the path I'm looking for.
I know that the following is valid SQL:
INSERT INTO [MyTable] ( [Col1], [Col2], [Col3] )
SELECT N'Val1', N'Val2', N'Val3';
But, is it at all possible to write something akin to this:
INSERT INTO [MyTable]
SELECT [Col1] = N'Val1', [Col2] = N'Val2', [Col3] = N'Val3';
By having the columns in the select statement, I'm able to do it all at once vs writing 2 separate lines. Obviously my idea doesn't work, I'm trying to figure out whether something similar is possible or I need to stick with the first one.
Much appreciated.
Best practice for insert statements is to specify the columns list in the insert clause, and for very good reasons:
It's far more readable. You know exactly what value goes into what column.
You don't have to provide values to nullable \ default valued columns.
You're not bound to the order of the columns in the table.
In case a column is added to the table, your insert statement might not break (It will if the newly added column is not nullable and doesn't have a default value).
In some cases, SQL Server demands you specify the columns list explicitly, like when identity_insert is set to on.
And in any case, the column names or aliases in the select clause of the insert...select statement does not have any effect as to what target columns the value column should go to. values are directed to target based only on their location in the statement.
I have three different values in my database that represent a null: an actual null, an empty string, and a string {x:Null}. This value appears across multiple columns.
{x:Null} is normalized on the web front-end, so all these values look exactly the same although they end up ordered differently in a sort. How can I write a query that will take these values and make them actual nulls across every column and every table?
Bonus points if you can tell me how to make sure these other empty values are always inserted as nulls going forward. (Disclaimer: I have no power to grant any actual bonus points. ;)
You can query the information_schema to get a list of all tables and columns with a string type.
SELECT table_name, column_name
FROM information_schema.columns
WHERE data_type IN ('text', 'character', 'character varying')
NOTE double check first what values data_type has, I'm not sure if it will be character or char or what.
Then I would write a small program to update each column in each table. Here it is sketched out in Perl.
while( my($table, $column) = $sth->fetch ) {
my $q_table = $dbh->quote($table);
my $q_column = $dbh->quote($column);
$dbh->do(q[
UPDATE `$q_table`
SET `$q_column` = NULL
WHERE `$q_column` = '{x:Null}'
OR `$q_column` = ''
]);
}
Be sure to SQL escape $table and $column as in my sample.
Going forward, you'll have to set CONSTRAINTS on each and every column. You can use the information_schema.columns to do this as well. Something like
ALTER TABLE `$q_table` ADD CHECK(`$q_column` NOT IN ('{x:Null}', ''))
You could use a trigger to change the values to NULL, but I don't like data stores that silently change basic data for application purposes.
For new columns and tables, you'll have to remember to add that constraint. Same caveats about data_type apply.
However, it's probably a bad idea to say that no column can ever be an empty string. You might want to be bit more selective.
Another thing to note: NULL is a funny thing, its not true and its not false. You might be better off deciding that an empty string is the thing to set empty values to.
I don't think this approach is maintainable. It's scribbling an application rule all over the data layer. What if you have some data that doesn't follow that rule? And it will have to be continuously maintained for any new data schema added. Perhaps instead you should put this at your ORM layer. Or write a few stored procedures to take care of this.
Using the information_schema.columns table, write a procedural language routine which iterates through all applicable tables and columns, executing an update... set *column* = NULL...where column in ('','{x:Null}'). for each eligible column.
As for inserting these values as NULL going forward, you would have to set triggers on your tables to intercept these values and replace them with NULL.
I don't think there is any query that would do this thing for every table and every column. In principle, what you want to do is
UPDATE table SET column=NULL WHERE column='' OR column='{x:Null}';
You could try selecting data from the pg_attribute and pg_class columns to get the names of the tables and names of the columns and then generating automatically the queries. Be sure to select only those columns that contain textual data.
What if somebody has entered a genuine string '{x:Null}'? You would then change it into NULL.
However, you have done a real mistake by letting the situation to be as bad as it's currently. You should always normalize data before putting it into a database.
In my table results from column work_time (interval type) display as 200:00:00. Is it possible to cut the seconds part, so it will be displayed as 200:00? Or, even better: 200h00min (I've seen it accepts h unit in insert so why not load it like this?).
Preferably, by altering work_time column, not by changing the select query.
This is not something you should do by altering a column but by changing the select query in some way. If you change the column you are changing storage and functional uses, and that's not good. To change it on output, you need to modify how it is retrieved.
You have two basic options. The first is to modify your select queries directly, using to_char(myintervalcol, 'HH24:MI')
However if your issue is that you have a common format you want to have universal access to in your select query, PostgreSQL has a neat trick I call "table methods." You can attach a function to a table in such a way that you can call it in a similar (but not quite identical) syntax to a new column. In this case you would do something like:
CREATE OR REPLACE FUNCTION myinterval_nosecs(mytable) RETURNS text LANGUAGE SQL
IMMUTABLE AS
$$
SELECT to_char($1.myintervalcol, 'HH24:MI');
$$;
This works on the row input, not on the underlying table. As it always returns the same information for the same input, you can mark it immutable and even index the output (meaning it can be run at plan time and indexed used).
To call this, you'd do something like:
SELECT myinterval_nosecs(m) FROM mytable m;
But you can then use the special syntax above to rewrite that as:
SELECT m.myinterval_nosecs FROM mytable m;
Note that since myinterval_nosecs is a function you cannot omit the m. at the beginning. This is because the query planner will rewrite the query in the former syntax and will not guess as to which relation you mean to run it against.
Trying to be lazy when looking at an example
SELECT realestate.address, realestate.parcel, s.sale_year, s.sale_amount,
FROM realestate INNER JOIN
dblink('dbname=somedb port=5432 host=someserver
user=someuser password=somepwd',
'SELECT parcel_id, sale_year,
sale_amount FROM parcel_sales')
AS s(parcel_id char(10),sale_year int, sale_amount int)
Is there a way of getting the AS section filled in from the table?
I'm copying data from tables of the same name and structure on different servers.
If I can get the structure to copy from the existing table, it will save me a lot of time
Thanks
Bruce
The answer is: No. See the doc:
Since dblink can be used with any query, it is declared to return record, rather than specifying any particular set of columns. This means that you must specify the expected set of columns in the calling query — otherwise PostgreSQL would not know what to expect.
http://www.postgresql.org/docs/9.1/static/contrib-dblink-function.html
Edit: by the way, for a table or a view, you can get the fields name and type in a first query:
select column_name
from information_schema.columns
where table_name = 'your_table_or_view';
You could then use it to fill the fields declaration.
Alexis
I am have two fields in my table:
One is Primary key auto increment value and second is text value.
lets say: xyzId & xyz
So I can easily insert like this
insert into abcTable(xyz) Values('34')
After performing above query it must insert these information
xyzId=1 & xyz=34
and for retrieving I can retrieve like this
select xyzId from abcTable
But for this I have to write down two operation. Cant I retrieve in single/sub query ?
Thanks
If you are on SQL Server 2005 or later you can use the output clause to return the auto created id.
Try this:
insert into abcTable(xyz)
output inserted.xyzId
values('34')
I think you can't do an insert and a select in a single query.
You can use a Store Procedures to execute the two instructions as an atomic operation or you can build a query in code with the 2 instructions using ';' (semicolon) as a separator betwen instructions.
Anyway, for select identity values in SQL Server you must check ##IDENTITY, SCOPE_IDENTITY and IDENT_CURRENT. It's faster and cleaner than a select in the table.