Archive for the ‘SQL DB Engine’ Category

Physical Join vs Logical Join in SQL Server

October 6, 2010 5 comments

Most of us know about JOINS in SQL Server and their types. But do we really know how they are interpreted in SQL Server internally. Today I found lot of informative, interesting and important sources regarding Logical and Physical joins in SQL Server (links below).

–> Classifying JOINS mainly into 2 types:

1. Logical Joins: These joins are simple joins that we apply in our SQL queries, like INNER JOIN, RIGHT/LEFT OUTER JOIN, CROSS JOIN, OUTER APPLY, etc.

2. Physical Joins: These are the joins that users don’t use/write in their SQL queries. Instead these are implemented inside SQL Server engine as operators or algorithms to implement the Logical Joins. Their types are Nested Loop, Merge and Hash.

For a particular SQL query when you try to view an Estimated Execution Plan or execute a query by selecting Actual Execution Plan, you can clearly see these Physical Joins under the Execution Plan tab in SSMS.

Reference for Logical Joins from one of my old posts:

–> Logical Joins:

– Inner/Outer/Cross:

–> Physical Joins:

– Nested Loop Joins:

– Merge Joins:

– Hash Joins:

SQL DBA – Collation Conflict in SQL Server

February 23, 2010 6 comments

Cannot resolve the collation conflict between “SQL_Latin1_General_CP437_BIN” and “SQL_Latin1_General_CP1_CI_AS” in the equal to operation.

Just few days back I came across this error when I tried to join two tables from 2 different databases. Didn’t get any clue for a few minutes so I googled up this error (Thanks google baba).

Collation is MS SQL Server is used for specifying the ordering of characters when you create or alter a table or create a domain. Collation settings, which include character set, sort order, and other locale-specific settings, are fundamental to the structure and function of Microsoft SQL Server databases. SQL Server uses them implicitly in SQL statements. To use them explicitly you need to override the default collation or the collation you specified when creating or altering the table or creating the domain. Collation can be applied for char, varchar, text, nchar, nvarchar, and ntext data types.

For example:

SELECT T1.EmployeeName, T2.DeptName
FROM ServerA.dbo.EmpTab T1
JOIN ServerB.dbo.DeptTab T2
ON T1.DeptCode = T2.DeptCode

There could be a possibility that both the servers use different Collations. If yes then you would get an error similar to then one I mentioned at the top of this topic. What you should do in this case?
1. You can alter the default Collation of either of the table columns/fields, but this could have a ripple effect for other tables currently in use with the altered table.
2. Use COLLATE DATABASE_DEFAULT keyword while matching the columns/fields, like:

SELECT T1.EmployeeName, T2.DeptName
FROM ServerA.dbo.EmpTab T1
JOIN ServerB.dbo.DeptTab T2

More Info:
Collation Types:
Alter Table:
Alter Database: