SQL Archives - Deep in the Code

2023.05.092023.05.09

Finding Column and Table Names in Amazon Redshift using SQL

Finding metadata regarding table and column names in Redshift can be daunting, especially if you do not know if the tables are internal or external. Redshift has a table called SVV_EXTERNAL_COLUMNS that is useful for looking at external tables, and there are scripts to look at internal tables as well.

The SQL code I wrote below combines these and should make it easier to find what you’re looking for.

select * from (
select t.table_schema as schemaname,
       t.table_name as tablename,
       c.column_name as columnname,
       t.table_type
from information_schema.tables t
inner join information_schema.columns c 
           on c.table_name = t.table_name 
           and c.table_schema = t.table_schema
where t.table_schema not in ('information_schema', 'pg_catalog')      
union all
select schemaname,
       tablename,
       columnname,
       'EXTERNAL TABLE' as table_type
from svv_external_columns)
where columnname ilike '%whatever column%'
and tablename ilike '%whatever table%'
and schemaname ilike '%whatever schema%'
order by schemaname, tablename, columnname;

I have found this to be useful on numerous occasions, especially since the Databaser Explorer in SQL Workbench/J does not yet seem to have a way to search for column names.

2022.06.272022.12.20

Testing Connectivity to Multiple SQL Server Linked Servers

As we are preparing to migrate some of our older SQL Server databases that are nearing their end of support to SQL Server 2019, one of the necessary tasks is determining what Linked Servers need to be established on the new servers to maintain the existing functionality for all of our users’ applications.

Testing the functionality of an individual Linked Server is simple enough – the built-in stored procedure sp_testlinkedserver will return a zero (0) if the link is good, and returns an exception if the link is not.

There is a stored procedure for listing all Linked Servers, but it doesn’t return the health of the link: sp_linkedservers.

How can these be combined to return only the Linked Servers that are currently up?

The solution I developed uses sp_linkedservers to populate a table variable, which is then used in a cursor. For each row in the table, using the sp_testlinkedserver stored proc, an active column is set to 1 if the server is up, and 0 if it is not.

DECLARE @LinkedServerName NVARCHAR(128), @Active BIT, @Message NVARCHAR(MAX)=''

DECLARE @Servers TABLE
(SRV_NAME NVARCHAR(128)
,SRV_PROVIDERNAME	NVARCHAR(128)
,SRV_PRODUCT	NVARCHAR(128)
,SRV_DATASOURCE	NVARCHAR(4000)
,SRV_PROVIDERSTRING	NVARCHAR(4000)
,SRV_LOCATION	NVARCHAR(4000)
,SRV_CAT	NVARCHAR(128)
,Active BIT DEFAULT 1)

INSERT INTO @Servers (SRV_NAME,SRV_PROVIDERNAME,SRV_PRODUCT,SRV_DATASOURCE,SRV_PROVIDERSTRING,SRV_LOCATION,SRV_CAT)
EXEC sp_linkedservers 

DECLARE servercursor CURSOR FOR 
	SELECT SRV_NAME, Active 
	FROM @Servers
	ORDER BY SRV_NAME

OPEN servercursor

FETCH NEXT FROM servercursor INTO @LinkedServerName, @Active
WHILE @@FETCH_STATUS=0
BEGIN
	DECLARE @isUp BIT = 0
		
	BEGIN TRY
		DECLARE @test INT
		EXEC @test = sp_testlinkedserver @LinkedServerName;
		IF @test = 0
			BEGIN
				SET @isUp = 1
			END
		ELSE
			BEGIN
				SET @isUp = 0
			END
	END TRY
	BEGIN CATCH			
		SET @isUp = 0;
	END CATCH
		
	IF @Active<>@isUp
	BEGIN
		UPDATE @Servers 
		SET Active=@isUp
		WHERE SRV_NAME=@LinkedServerName

		SET @Message = @Message + 'Active bit on server ' + @LinkedServerName + ' was set to ' + CONVERT(VARCHAR,@isUp) + '.' + CHAR(10) + CHAR(13)
	END		
	FETCH NEXT FROM servercursor INTO @LinkedServerName, @Active
END

CLOSE servercursor
DEALLOCATE servercursor

SELECT SRV_NAME
FROM @Servers
WHERE Active=1
ORDER BY SRV_NAME

PRINT @Message

Running this code will return an alphabetically ordered list of active linked servers.

2018.04.202022.12.20

Comparing Multiple Patterns with LIKE Operator

I really like the LIKE operator! (See what I did there?)

I use it on a daily basis. It does have one frustrating shortcoming, however. Unlike the IN operator (where a list is used for comparison), you can only compare one pattern with each LIKE statement.

If you have only a few patterns, this isn’t too much of a problem.

SELECT * FROM tbl_ThatTableOverThere
WHERE ThisColumn LIKE '%pattern1%'
OR ThisColumn LIKE '%pattern2%'
OR ThisColumn LIKE '%pattern3%'
-- [...]
OR ThisColumn LIKE '%patternX%';

But what if X is a very large number? Hundreds? Thousands? Maybe even millions?

I wrote the code below to allow for multiple patterns to be compared. The patterns are inserted into a table variable (@patterns) and are selected using a fast forward cursor. Unless the number of patterns is truly astronomical, this should suffice.

USE DatabaseName
GO
SET NOCOUNT ON
DECLARE @random INT, @upper INT, @lower INT, @randomTable NVARCHAR(20)
DECLARE @SQL NVARCHAR(MAX)
DECLARE @whereClause NVARCHAR(MAX)
DECLARE @sourceTable NVARCHAR(255)
DECLARE @columnName NVARCHAR(255)
DECLARE @pattern NVARCHAR(255)
DECLARE @patterns TABLE (pattern NVARCHAR(255) NOT NULL)
DECLARE @deleteTempTable NVARCHAR(MAX)

/* These variables should remain unchanged unless the name of the
temporary table needs to have different text or range of random numbers
for the suffix of the temp table.
[Reference: https://stackoverflow.com/questions/14756780/creating-a-temporary-table-name-with-a-randomly-generated-number] */
SET @lower = 1
SET @upper = 9999999
SELECT @random = ROUND(((@upper - @lower) * RAND() + @lower),0)
SELECT @randomTable = '##search'+CAST(@random AS NVARCHAR(20))+''

-- Name of source table holding data for comparison
SET @sourceTable = 'tbl_Name'

-- Column that is being compared with patterns using LIKE operator
SET @columnName = 'ColumnName'

-- Additional WHERE clause may be specified here
SET @whereClause = '' -- e.g., 'AND AnyColumnName LIKE ''%b%''' 

-- Insert patterns into table variable below
INSERT INTO @patterns VALUES
('%pattern1%')
-- ,('%pattern2%')
-- ,(%pattern3%) and so on

-- Deletes temp table if it already exists
SET @deleteTempTable = 'IF object_id(''tempdb..' + @randomTable + ''') IS NOT NULL DROP TABLE ' + @randomTable
EXEC(@deleteTempTable)

-- This line creates the temporary table using the same schema as the source table without inserting data.
-- The UNION ALL operator prevents the IDENTITY property from being applied to any of the columns in the temp table.
-- [Reference: https://dba.stackexchange.com/questions/916/how-do-i-copy-a-table-with-select-into-but-ignore-the-identity-property]
EXEC('SELECT TOP 0 * INTO ' + @randomTable + ' FROM ' + @sourceTable + ' UNION ALL SELECT TOP 0 * FROM ' + @sourceTable)

SET @SQL = 'INSERT INTO ' + @randomTable + CHAR(13) + 'SELECT TOP 0 * FROM ' + @sourceTable

DECLARE patternsCursor CURSOR FAST_FORWARD FOR
SELECT pattern FROM @patterns

OPEN patternsCursor
FETCH NEXT FROM patternsCursor INTO @pattern

WHILE @@FETCH_STATUS = 0
BEGIN
	IF LEN(@pattern) > 0
	BEGIN
		--Note: UNION operator (as opposed to UNION ALL) is used here to prevent duplicate rows from being inserted into table variable
		--[Reference: https://www.techonthenet.com/sql/union_all.php]
		SET @SQL = @SQL + CHAR(13) + 'UNION' + CHAR(13) + 'SELECT * FROM ' + @sourceTable + CHAR(13) + 'WHERE ' + @columnName + ' LIKE ''' + @pattern + ''''
		IF LEN(@whereClause) > 0 SET @SQL = @SQL + CHAR(13) + @whereClause
	END

	FETCH NEXT FROM patternsCursor INTO @pattern
END
CLOSE patternsCursor
DEALLOCATE patternsCursor
SET @SQL = @SQL + ';'

-- Prints final SQL statement on Results tab
SELECT @SQL
SET NOCOUNT OFF
-- Executes SQL Statement showing number of rows inserted on Messages tab
EXEC(@SQL)
SET NOCOUNT ON

-- Executes SQL Statement showing rows in Results tab
EXEC('SELECT * FROM ' + @randomTable + ';')

-- Deletes temp table
EXEC(@deleteTempTable)