I have a very simple select statement like
SELECT Column-Name
FROM Object
WHERE ID = 123
where the Column-Name is dynamically generated. Is there a possibility to get an empty string if column not exists?
This is a huge hack for SQL Server
SQL is batch based: when submitted to the database engine, the whole batch is parse and a plan compiled. If a column or object does not exist, you'll get an error.
You can test the metadata to see if it exists (COLUMNPROPERTY) then use EXEC to have the SELECT in an separate batch
IF COLUMNPROPERTY(OBJECT_ID('Object'), 'Column-Name', 'ColumnID') IS NOT NULL
EXEC ('SELECT Column-Name FROM Object WHERE ID = 123')
ELSE
SELECT '' AS Column-Name;
Personally, I'd never expect this to be in production code or running on my database servers.
Why not do a desc object or something similar and validate if the column name exists before firing the query?
Otherwise you will end up doing funny things to work around the sql error
Related
I'm building a Stored Procedure meant to be run daily. Due to how the data is provided me, some days some of the columns necessary for the output are not in the file that's imported to a temporary table.
For the output, the value can be null/empty, but the actual column has to be there.
I've tried the following code:
select
case
when exists(
select * from INFORMATION_SCHEMA.COLUMNS
where TABLE_NAME = 'tableName' and COLUMN_NAME = 'ABC'
) then ABC
else ''
end
as 'XYZ' from tableName
I know for a fact that in the tests I'm running the column DOES NOT exist, so I'm expecting the SELECT statment to simply return an empty string for column XYZ.
However when running such SELECT statement, I get the following error:
Invalid column name 'ABC'.
Since I know from the start that the column ABC does not exist, I was expecting that the EXISTS(...) would evaluate to FALSE and jump straight to the ELSE statement. However it seems that the column name is still evaluated.
How can I get around this?
I believe i have solved my troubles thanks to the workaround provided by #MartinSmith
(https://dba.stackexchange.com/questions/66741/why-cant-i-use-a-case-statement-to-see-if-a-column-exists-and-not-select-from-i/66755#66755)
I ended up using an alternative version of the workaround provided, linked by the poster of #MartinSmith's solution, but the results seem to be what i was expecting.
Regarding the Only one expression can be specified in the select list when the subquery is not introduced with EXISTS error, it was being caused by me trying to SELECT two columns in the subquery. Solution came from here: https://stackoverflow.com/a/7684626/18191554
Now, my code is as follows:
select
.
.
.
.
(select (SELECT [ABC]
from dbo.tableName
where ID = temp.ID])
from (select '' as [ABC]) as dummy)
as 'XYZ',
.
.
.
from tableName
where ABC is the column that may or may not exist.
Thanks to everyone involved
I am in the process of porting over some data from a SQL environment to my MongoDB backend. I'm familiar with using a NULL check with your SELECT statement, like so:
SELECT * FROM clients WHERE note is not NULL ORDER BY id_number
... but in this old SQL database table I'm noticing a lot of rows where the value is not null, it's simply empty. It would be pointless to port these across. So what would it look like to prevent pulling those over -- in terms of the SELECT statement syntax?
To clarify "note" values are of type varchar. In JavaScript I would just guard against an empty string " ". Is there a way to do this with a SQL statement?
Something like that :
SELECT * FROM clients
WHERE note is not NULL
AND TRIM(note) <> ''
ORDER BY id_number;
I want to do something along the lines of:
SELECT some_things
FROM `myproject.mydataset.mytable_#suffix`
But this doesn't work because the parameter isn't expanded inside the table name.
This does work, using wildcard tables:
SELECT some_things
FROM `myproject.mydataset.mytable_*`
WHERE _TABLE_SUFFIX = #suffix
However, it has some problems:
If I mistype the parameter, this query silently returns zero rows, rather than yelling at me loudly.
Query caching stops working when querying with a wildcard.
If other tables exist with the mytable_ prefix, they must have the same schema, even if they don't match the suffix. Otherwise, weird stuff happens. It seems like BigQuery either computes the union of all columns, or takes the schema of an arbitrary table; it's not documented and I didn't look at it in detail.
Is there a better way to query a single table whose name depends on a query parameter?
Yes, you can, here's a working example:
DECLARE tablename STRING;
DECLARE tableQuery STRING;
##get list of tables
CREATE TEMP TABLE tableNames as select table_name from nomo_nausea.INFORMATION_SCHEMA.TABLES where table_name not in ('_sdc_primary_keys', '_sdc_rejected', 'fba_all_order_report_data');
WHILE (select count(*) from tableNames) >= 1 DO
SET tablename = (select table_name from tableNames LIMIT 1);
##build dataset + table name
SET tableQuery = CONCAT('nomo_nausea.' , tablename);
##use concat to build string and execute
EXECUTE IMMEDIATE CONCAT('SELECT * from `', tableQuery, '` where _sdc_deleted_at is not null');
DELETE FROM tableNames where table_name = tablename;
END WHILE;
In order to answer your stated problems:
Table scanning happens in FROM clause, in WHERE clause happens filtering [1] thus if WHERE condition is not match an empty result would be returned.
"Currently, Cached results are not supported when querying with wildcard" [2].
"BigQuery uses the schema for the most recently created table that matches the wildcard as the schema" [3]. What kind of weird stuff you have faced in your use case? "A wildcard table represents a union of all the tables that match the wildcard expression" [4].
In BigQuery parameterized queries can be run, But table names can not be parameterized [5]. Your wildcard solution seems to be the only way.
You can actually use tables as parameters if you use the Python API, but it's not documented yet. If you pass the tables as parameters through a formatted text string vs. a docstring, your query should work.
SQL example:
sql = "SELECT max(_last_updt) FROM `{0}.{1}.{2}` WHERE _last_updt >= TIMESTAMP(" +
"CURRENT_DATE('-06:00'))".format(project_id, dataset_name, table_name)
SQL in context of Python API:
bigquery_client = bigquery.Client() #setup the client
query_job = bigquery_client.query(sql) #run the query
results = query_job.result() # waits for job to complete
for row in results:
print row
I have the following query in a stored procedure in SQL server:
SELECT TLI.LESNumber
,COUNT(TLT.PL)
INTO #PWCM
FROM #tmpLESImport TLI
INNER JOIN tbl_LES L
on TLI.LESNumber=L.NUMB
WHERE ISNULL(L.DELT_FLAG,0)=0
AND L.SCHL_PK=#SCHL_PK
AND TLI.PL IS NOT NULL
AND LEN(TLI.PL)>0
GROUP BY LESNumber
HAVING COUNT(PL)>1
When the query is run I get the following error:
An object or column name is missing or empty. For SELECT INTO statements, verify each column has a name. For other statements, look for empty alias names. Aliases defined as "" or [] are not allowed. Change the alias to a valid name.
Can anyone tell me why? #PWCM does not appear anywhere until this query.
When you SELECT INTO a table, it creates the table (in this case, a temp table). In order to create a table, each column needs a name, which your count column does not. You just need to give it a name:
SELECT TLI.LESNumber,COUNT(TLT.PL) [NumRecords]
INTO #PWCM
FROM #tmpLESImport TLI
...
I had this error for this query
SELECT
CASE WHEN COALESCE([dbo].[my-table].[field],"") = '...' THEN 'A'
WHEN COALESCE([dbo].[my-table].[field],"") = '...' THEN 'B'
...
END AS aaa INTO ##TEMPTABLE
FROM [dbo].[my-table]
Turns out I had to change the "" inside the COALSCE into ''.
Solved it for me
I just came across this, and the core reason was actually related to an errant set of brackets in the code which made the engine think there was a missing alias. Something along the lines of:
select *
from SOME_TABLE
where x = 1
[]
A stringified version of the query included a parameter list for logging, but that was being issued as the query instead of the actual query object. Deleting [] at the end resolved it.
I had created the table with 200 columns and i had inserted data
Now i need to check that specific 100 columns in one row are filled or not,how can we check this using mysql query .the primary key is defined .please help me out how to resolve this.
select * from tablename where column1 != null or column2 != null ......
That is a lot of columns so at the risk of being mysql server version specific you can use the information schema to get the column names and then write a SQL procedure or something in your chosen shell / language that iterates over them performing a test.
select distinct COLUMN_NAME as 'Field', IS_NULLABLE from information_schema.columns where TABLE_SCHEMA="YourDatabase" and TABLE_NAME="YourTableName" and TABLE_NAME not like "%view%" escape '!' ;
The example above will tell you the column name as "Field" and tell you if it can hold a NULL. Having the field name may give you a better way of automating a field name specific test.