Get every n-th record using SQL Server

First, a few things you should already know:
Modulus Returns the remainder of one number divided by another.
SELECT 32/10 -> 3
SELECT 32%10 -> 2

To return the n-th row only of a table, use:

WITH myTableWithRows AS (
    SELECT (ROW_NUMBER() OVER (ORDER BY myTable.SomeField)) as row,*
    FROM myTable)
SELECT * FROM myTableWithRows WHERE row = [value of n]

To return every n-th row in a table, use:

WITH myTableWithRows AS (
    SELECT (ROW_NUMBER() OVER (ORDER BY myTable.SomeField)) as row,*
    FROM myTable)
SELECT * FROM myTableWithRows WHERE [row]%[value of n] = 0

Example:

Return every 10th event from an events table:

WITH tableEvents AS (
    SELECT (ROW_NUMBER() OVER (ORDER BY EventDate)) as row,*
    FROM tblEvents
	Where Month(EventDate) = 7)
SELECT * FROM tableEvents WHERE [row]%10= 0
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How to check if a constraint exists

In SQL Server, you can check if a constraint exists using the following script:

SELECT 
    OBJECT_NAME(OBJECT_ID) AS NameofConstraint
        ,SCHEMA_NAME(schema_id) AS SchemaName
        ,OBJECT_NAME(parent_object_id) AS TableName
        ,type_desc AS ConstraintType
    FROM sys.objects
    WHERE type_desc LIKE '%CONSTRAINT'
        AND OBJECT_NAME(OBJECT_ID)='[constraint name here]'

or by running one of the following commands:

SELECT * FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS where TABLE_Name = '[TableName]'

SELECT *
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS
WHERE CONSTRAINT_NAME='[constraint name here]'

--Returns one row for each FOREIGN KEY constrain
SELECT *
FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS
WHERE CONSTRAINT_NAME='[constraint name here]'

--Returns one row for each CHECK constraint
SELECT *
FROM INFORMATION_SCHEMA.CHECK_CONSTRAINTS
WHERE CONSTRAINT_NAME='[constraint name here]'

Enable and disable all indexes in a database

Disabling indexes is a good idea when it comes to loading large quantities of data, but… the big problem is clustered indexes. If you disable a clustered index, you’ve disabled the entire table.

Several options suggest themselves, and none of them are simple.

1) Loop through the system views (sys.indexes), extract the table and index name, generate and execute dynamic SQL to disable the index. Have an “undo” routine to re-enable them. (Be wary–was it a unique index or a unique constraint?) This, alas, only works if you do not use clustered indexes. Good luck with that.

2) As for 1, but skip any clustered indexes. When you load data, make sure it gets loaded in (clustered index) sequential order, otherwise you’ll have poor load times and fragmented tables. (If you data providers are like mine, good luck with that one, too.)

3) Create tables in your database containing definitions of the indexes on your “loading” tables. Build a routine that loops through them and drops all the indexes (clustered indexes last). This will be fast if you truncate the tables first. Load your data, then loop through and recreate the indexes from scratch (clustered first). Use table partitioning to make less horrible on the rest of the system (e.g. do all the above on the “loading” tables, then use partition switching to move the loaded data into your “live” tables). It took me no little time to build such a system, but it can and will work.

Disable script:

SELECT 'ALTER INDEX ' + QUOTENAME(I.name) + ' ON ' +  QUOTENAME(SCHEMA_NAME(T.schema_id))+'.'+ QUOTENAME(T.name) + ' DISABLE' 
FROM sys.indexes I
INNER JOIN sys.tables T ON I.object_id = T.object_id
WHERE I.type_desc = 'NONCLUSTERED'
AND I.name IS NOT NULL
AND I.is_disabled = 0

 

Enable script:

SELECT 'ALTER INDEX ' + QUOTENAME(I.name) + ' ON ' +  QUOTENAME(SCHEMA_NAME(T.schema_id))+'.'+ QUOTENAME(T.name) + ' REBUILD' 
FROM sys.indexes I
INNER JOIN sys.tables T ON I.object_id = T.object_id
WHERE I.type_desc = 'NONCLUSTERED'
AND I.name IS NOT NULL
AND I.is_disabled = 1

See currently running queries in SQL Server 2012

SELECT r.start_time [Start Time],session_ID [SPID],
DB_NAME(database_id) [Database],
SUBSTRING(t.text,(r.statement_start_offset/2)+1,
CASE WHEN statement_end_offset=-1 OR statement_end_offset=0
THEN (DATALENGTH(t.Text)-r.statement_start_offset/2)+1
ELSE (r.statement_end_offset-r.statement_start_offset)/2+1
END) [Executing SQL],
Status,command,wait_type,wait_time,wait_resource,
last_wait_type
FROM sys.dm_exec_requests r
OUTER APPLY sys.dm_exec_sql_text(sql_handle) t
WHERE session_id != @@SPID — don’t show this query
AND session_id > 50 — don’t show system queries
ORDER BY r.start_time

This query is great at finding locking queries like the one below:

To fix this, either add “(NOLOCK)” table hint to allow the select to run while the update is happening or kill the offending process using “KILL” command followed by the SPID

Script all indexes for all tables in a database (SQL Script)

The need often arises to create or recreate the indexes for all tables in a database, especially in development and testing scenarios. This article presents a script to generate Index Creation Scripts for all tables in a database usingTransact-SQL (T-SQL).

SELECT ' CREATE ' +  
    CASE WHEN I.is_unique = 1 THEN ' UNIQUE ' ELSE '' END  +   
    I.type_desc COLLATE DATABASE_DEFAULT +' INDEX ' +    
    I.name  + ' ON '  +   
    Schema_name(T.Schema_id)+'.'+T.name + ' ( ' +  
    KeyColumns + ' )  ' +  
    ISNULL(' INCLUDE ('+IncludedColumns+' ) ','') +  
    ISNULL(' WHERE  '+I.Filter_definition,'') + ' WITH ( ' +  
    CASE WHEN I.is_padded = 1 THEN ' PAD_INDEX = ON ' ELSE ' PAD_INDEX = OFF ' END + ','  +  
    'FILLFACTOR = '+CONVERT(CHAR(5),CASE WHEN I.Fill_factor = 0 THEN 100 ELSE I.Fill_factor END) + ','  +  
    -- default value  
    'SORT_IN_TEMPDB = OFF '  + ','  +  
    CASE WHEN I.ignore_dup_key = 1 THEN ' IGNORE_DUP_KEY = ON ' ELSE ' IGNORE_DUP_KEY = OFF ' END + ','  +  
    CASE WHEN ST.no_recompute = 0 THEN ' STATISTICS_NORECOMPUTE = OFF ' ELSE ' STATISTICS_NORECOMPUTE = ON ' END + ','  +  
    -- default value   
    ' DROP_EXISTING = ON '  + ','  +  
    -- default value   
    ' ONLINE = OFF '  + ','  +  
   CASE WHEN I.allow_row_locks = 1 THEN ' ALLOW_ROW_LOCKS = ON ' ELSE ' ALLOW_ROW_LOCKS = OFF ' END + ','  +  
   CASE WHEN I.allow_page_locks = 1 THEN ' ALLOW_PAGE_LOCKS = ON ' ELSE ' ALLOW_PAGE_LOCKS = OFF ' END  + ' ) ON [' +  
   DS.name + ' ] '  [CreateIndexScript]  
FROM sys.indexes I    
 JOIN sys.tables T ON T.Object_id = I.Object_id     
 JOIN sys.sysindexes SI ON I.Object_id = SI.id AND I.index_id = SI.indid    
 JOIN (SELECT * FROM (   
    SELECT IC2.object_id , IC2.index_id ,   
        STUFF((SELECT ' , ' + C.name + CASE WHEN MAX(CONVERT(INT,IC1.is_descending_key)) = 1 THEN ' DESC ' ELSE ' ASC ' END 
    FROM sys.index_columns IC1   
    JOIN Sys.columns C    
       ON C.object_id = IC1.object_id    
       AND C.column_id = IC1.column_id    
       AND IC1.is_included_column = 0   
    WHERE IC1.object_id = IC2.object_id    
       AND IC1.index_id = IC2.index_id    
    GROUP BY IC1.object_id,C.name,index_id   
    ORDER BY MAX(IC1.key_ordinal)   
       FOR XML PATH('')), 1, 2, '') KeyColumns    
    FROM sys.index_columns IC2    
    --WHERE IC2.Object_id = object_id('Person.Address') --Comment for all tables   
    GROUP BY IC2.object_id ,IC2.index_id) tmp3 )tmp4    
  ON I.object_id = tmp4.object_id AND I.Index_id = tmp4.index_id   
 JOIN sys.stats ST ON ST.object_id = I.object_id AND ST.stats_id = I.index_id    
 JOIN sys.data_spaces DS ON I.data_space_id=DS.data_space_id    
 JOIN sys.filegroups FG ON I.data_space_id=FG.data_space_id    
 LEFT JOIN (SELECT * FROM (    
    SELECT IC2.object_id , IC2.index_id ,    
        STUFF((SELECT ' , ' + C.name  
    FROM sys.index_columns IC1    
    JOIN Sys.columns C     
       ON C.object_id = IC1.object_id     
       AND C.column_id = IC1.column_id     
       AND IC1.is_included_column = 1    
    WHERE IC1.object_id = IC2.object_id     
       AND IC1.index_id = IC2.index_id     
    GROUP BY IC1.object_id,C.name,index_id    
       FOR XML PATH('')), 1, 2, '') IncludedColumns     
   FROM sys.index_columns IC2     
   --WHERE IC2.Object_id = object_id('Person.Address') --Comment for all tables    
   GROUP BY IC2.object_id ,IC2.index_id) tmp1    
   WHERE IncludedColumns IS NOT NULL ) tmp2     
ON tmp2.object_id = I.object_id AND tmp2.index_id = I.index_id    
WHERE I.is_primary_key = 0 AND I.is_unique_constraint = 0  
--AND I.Object_id = object_id('Person.Address') --Comment for all tables  
--AND I.name = 'IX_Address_PostalCode' --comment for all indexes  

 

List all tables in the database along with their row counts

There is a catalog view that will give us this information. The benefit of this approach is that it uses figures that have already been calculated by SQL Server so it should run very quickly. Here is the script :

SELECT T.name TableName,i.Rows NumberOfRows
FROM sys.tables T
JOIN sys.sysindexes I ON T.OBJECT_ID = I.ID
WHERE indid IN (0,1)
ORDER BY i.Rows DESC,T.name

The script uses the clustered index to source the information (where indid=1). Tables that don’t have a clustered index are stored as a heap but still have a row in sysindexes (where indid=0) containing the rowcount. There is one important caveat, though, the figures may not be entirely accurate ! This is because SQL Server doesn’t always keep this bang up to date. Fortunately there is a DBCC command which updates these figures. Just run this command before the script above to make sure the figures are accurate :
DBCC UPDATEUSAGE(0)