This code results in timeout exception
String City_Code = null;
var result = (from r in myContext.TableA
where ( City_Code == r.City_Code )
select r ).ToList();
while this code will return quickly
String City_Code = null;
var result = (from r in myContext.TableA
where ( r.City_Code == City_Code )
select r ).ToList();
The difference is in the order of operands of the equality. The field City_Code of table TableA is of type nvarchar(20).
What is a possible reason for that???
update: The generated tsql for the 2 queries are identical except for that part: the first one has the condition " #p__linq__2 = [Extent1].[City_Code])" while the second has " [Extent1].[City_Code] = #p__linq__2)"
The value of #p_linq_2 when I notice the time difference is NULL. If I run the queries in ssms with NULL in place of #p_linq_2 they respond both very quickly
Related
I would need to write a SQL question based on conditions:
in Condition 1:
SELECT
*
FROM
table_1
WHERE
col_1 IS NULL
AND col_2 IS NOT NULL
and in Condition 2:
SELECT
*
FROM
table_1
WHERE
col_1 IS NULL
How would I be able to achieve this easily in Python? I know I can do filters later on but that's not super efficient as it should be.
The solution used in many tools: Make initial query with dummy TRUE WHERE clause, then depending on conditions can be concatenated with additional filters like this (simplified code):
query = 'select * from table where 1 = 1' # WHERE with dummy TRUE condition
# it can be WHERE TRUE
condition1 = True; # if both conditions are False, query will be without filters
condition2 = True;
filter1='Col1 is not null'
filter2='Col2 is not null'
if condition1:
query = query+' and '+ filter1
if condition2:
query = query+' and '+ filter2
print(query)
Result:
select * from table where 1 = 1 and Col1 is not null and Col2 is not null
More elegant solution using pypika - python query builder. You can build the whole query including table, fields, where filters and joins:
from pypika import Query, Table, Field
table = Table('table')
q = Query.from_(table).select(Field('col1'), Field('col2'))
condition1 = True;
condition2 = True;
if condition1:
q = q.where(
table.col1.notnull()
)
if condition2:
q = q.where(
table.col2.notnull()
)
print(q)
Result:
SELECT "col1","col2" FROM "table" WHERE NOT "col1" IS NULL AND NOT "col2" IS NULL
I have this autoquery implementation
var q = AutoQuery.CreateQuery(request, base.Request).SelectDistinct();
var results = Db.Select<ProductDto>(q);
return new QueryResponse<ProductDto>
{
Offset = q.Offset.GetValueOrDefault(0),
Total = (int)Db.Count(q),
Results = results
};
The request has some joins:
public class ProductSearchRequest : QueryDb<GardnerRecord, ProductDto>
, ILeftJoin<GardnerRecord, RecordToBicCode>, ILeftJoin<RecordToBicCode, GardnerBicCode>
{
}
The records gets returned correctly but the total is wrong. I can see 40,000 records in database but it tells me there is 90,000. There is multiple RecordToBicCode for each GardnerRecord so it's giving me the number of records multiplied by the number of RecordToBicCode.
How do I match the total to the number of GardnerRecord matching the query?
I am using PostgreSQL so need the count statement to be like
select count(distinct r.id) from gardner_record r etc...
Dores OrmLite have a way to do this?
I tried:
var q2 = q;
q2.SelectExpression = "select count(distinct \"gardner_record\".\"id\")";
q2.OrderByExpression = null;
var count = Db.Select<int>(q2);
But I get object reference not set error.
AutoQuery is returning the correct total count for your query of which has left joins so will naturally return more results then the original source table.
You can perform a Distinct count with:
Total = Db.Scalar<long>(q.Select(x => Sql.CountDistinct(x.Id));
While using ServiceStack.OrmLite 3.9.70.0, and following some of the examples from the ServiceStack.OrmLite wiki.
I am trying to select rows where the LastActivity date month = 1.
I keep getting the error:
{"variable 'pp' of type 'Author' referenced from scope '', but it is not defined"}
LastActivity is a nullable DateTime, defind like:
public DateTime ? LastActivity { get; set;}
I have tried:
db.Select<Author>(q => q.LastActivity.Value.Month == 1);
AND
var visitor = db.CreateExpression<Author>();
db.Select<Author>(visitor.Where(q => q.LastActivity.Value.Month == 1));
AND
SqlExpressionVisitor<Author> ev = OrmLiteConfig.DialectProvider.ExpressionVisitor<Author>();
db.Select<Author>(ev.Where(q => q.LastActivity.Value.Month == 1));
AND
var predicate = ServiceStack.OrmLite.PredicateBuilder.True<Author>();
predicate = predicate.And(q => q.LastActivity.Value.Month == 1);
db.Select<Author>(predicate);
I am trying to avoid using a sql string in the select because I like the compile time checking of the field names and types.
do a less than and more than on the date field IE
LastActivity >= variableThatHoldsStartDateOfMonth && LastActivity <= VariableThatHoldsLastDayOfMOnth.
This will give you results for the whole month
Assume you have a table with a nullable varchar column. When you try to filter the table, you would use (pFilter is parameter):
var filter = pFilter;
var dataContext = new DBDataContext();
var result = dataContext.MyTable.Where(x=>x.MyColumn == filter).ToList();
Now, what if there is a keyword that means "All Nulls". The code would look like:
var filter = pFilter != "[Nulls]" ? pFilter : null;
var dataContext = new DBDataContext();
var result = dataContext.MyTable.Where(x=>x.MyColumn == filter).ToList();
But this doesn't work. Apparently, a String with value of null is... not null?
However, what do work is this code:
var filter = pFilter != "[Nulls]" ? pFilter : null;
var dataContext = new DBDataContext();
var result = dataContext.MyTable.Where(x=>x.MyColumn == filter || (filter == null && x.MyColumn == null)).ToList();
The workaround did not convinced me, that's why my question is: What is the best way to deal with nullable string columns in LinqToSql?
Use String.Equals that will make LINQ handle null appropriately on the generated SQL query
var result = dataContext.MyTable
.Where(x => String.Equals(x.MyColumn, filter))
.ToList();
Edit:
If you use == LINQ will generate the query for the general case WHERE [column] = #parameter but on SQL NULL does not match NULL, the proper way to test for NULL is [column] IS NULL.
With String.Equals LINQ has enough information to translate the method to the appropiate sentence in each case, what means:
if you pass a non-null string it will be
WHERE ([column] IS NOT NULL) AND ([column] = #parameter)
and if it is null
WHERE [column] IS NULL
I'm trying to using the Subsonic Fluent query interface to create a simple inner join between two tables:
[SearchJobResults]
PK SearchJobResultId int
Name string
Desc string
[ParseResults]
PK ParseResultId int
Name string
SearchJobResultId int
There is a 1 to 1 relationship between these tables.
Keep in mind, I'm not using ActiveRecord. I have classes for ParseResult and SearchJobResult that work fine.
IDataProvider p = ProviderFactory.GetProvider("DemacDB");
SqlQuery query = new SqlQuery(p);
var q = new Select(p).From("ParseResults")
.InnerJoin<SearchJobResult>("SearchJobResultId","SearchJobResultId").GetRecordCount();
This code throws an exception:
Test method Models.SearchTests.TestSubsonicQueryMethods threw exception: System.InvalidOperationException: Don't know which column to join to - can't find column SearchJobResultId in table ParseResults.
I've looked at the source code for SubSonic to see where this execption is coming from:
private void CreateJoin<T>(string fromColumnName, string toColumnName, Join.JoinType type)
{
//see if we can find the table
var toTable = _provider.FindOrCreateTable(typeof(T));
//the assumption here is that the FromTable[0] is the table to join from
if(FromTables.Count == 0)
throw new InvalidOperationException("Can't join if there's no table to join to - make sure to use From() before InnerJoin");
if(toTable == null)
throw new InvalidOperationException("Can't find the table for this type. Try using the Column instead");
var fromColumn = FromTables[0].GetColumn(fromColumnName);
if(fromColumn == null)
throw new InvalidOperationException("Don't know which column to join to - can't find column " + fromColumnName + " in table " + FromTables[0].Name);
var toColumn = toTable.GetColumn(toColumnName);
if(toColumn == null)
throw new InvalidOperationException("Don't know which column to join to - can't find column " + toColumnName + " in table " + toTable.Name);
CreateJoin(fromColumn, toColumn, Join.JoinType.Inner);
}
I've tried using Aliases but that fails. Additionally, if I just do a simple query like this it works fine:
var d = new Select(p).From("ParseResults").GetRecordCount();
Turns out you need to use the Typed T overloads of From/Join to get this working.
var b = new Select(p).From<ParseResult>().InnerJoin<SearchJobResult>("SearchJobResultId", "SearchJobResultId").GetRecordCount();
Works as the FromTables collection in Subsonic now correctly gets enumerated because it's reading the type from the actual object and not the DB.