TSQL Learning

This post is based on very good reading from Article T-SQL Misconceptions - JOIN ON vs. WHERE. These are new learning's for me


1. For Readability - Refrain from using search arguments in the ON clause, and use the WHERE clause instead
2. When Outer Join is Converted to Inner Join - Adding references to the table in the right side of a JOIN to the WHERE clause will convert the OUTER JOIN to an INNER JOIN. I'm trying out Example Code to reproduce the scenario. This Shows the behavior when Left Join in Query is Converted to Inner Join during Execution.


Step 1 - Create Tables

CREATE TABLE Table1(Col1 INT IDENTITY(1,1) PRIMARY KEY, Col2 VARCHAR(40), Col3 VARCHAR(50))

CREATE TABLE Table2(Col1 INT IDENTITY(1,1) PRIMARY KEY, Col2 VARCHAR(40), Col3 VARCHAR(50))

Step 2 - Populate Data

DECLARE @J INT

 SET @J = 1
 WHILE 1 = 1
 BEGIN
 IF @J < 1000
     INSERT INTO Table1(Col2, Col3)
     VALUES((CONVERT(VARCHAR(20),@J)+'VALUE'), (CONVERT(VARCHAR(20),@J)+'VALUE'))
     INSERT INTO Table2(Col2, Col3)
     VALUES((CONVERT(VARCHAR(20),@J)+'VALUE'), (CONVERT(VARCHAR(20),@J)+'VALUE'))
     SET @J = @J + 1
     IF @J > 1000
     BREAK;
 END


Step 3 - Run Queries

SELECT

 N1.Col1, N2.Col2
 FROM Table1 N1
 LEFT JOIN Table2 N2
 ON N1.Col1 = N2.Col1
 WHERE N2.Col1 = 100
 

SELECT

 N1.Col1, N2.Col2
 FROM Table1 N1
 LEFT JOIN Table2 N2
 ON N1.Col1 = N2.Col1
 WHERE N1.Col1 = 120


Step 4 - Execution Plan

Happy Learning!!!