--Make the System ignore the constraint
ALTER TABLE PK_ConstraintsTableName NOCHECK CONSTRAINT ALL

--Now Delete
DELETE
FROM TableName
WHERE WhereClause

--Put the contraints back
ALTER TABLE PK_ConstraintsTableName CHECK CONSTRAINT ALL

SQL  - Using Case Statement To Update A Table: 

Here is an example of what different join produce in SQL.


Create and populate two tables:
CREATE TABLE #t1 (ID INT, name VARCHAR(10), val1 INT, val2 INT)
CREATE TABLE #t2 (ID INT, name VARCHAR(10), val1 INT, val2 INT)

INSERT #t1  (ID , name , val1 , val2 )
SELECT 1, 'aaa01', 10, 100
UNION ALL
SELECT 2, 'bbb01', 20, 200
UNION ALL
SELECT 3, 'aaa02', 30, 300
UNION ALL
SELECT 4, 'bbb02', 40, 400


INSERT #t2  (ID , name , val1 , val2 )
SELECT 3, 'ccc03', 10, 100
UNION ALL
SELECT 4, 'ddd03', 20, 200
UNION ALL
SELECT 5, 'ccc04', 30, 300
UNION ALL
SELECT 6, 'ddd04', 40, 400

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LEFT JOIN: 

SELECT #t1.*, #t2.*
FROM #t1
LEFT JOIN #t2 ON #t1.id = #t2.id

ID name val1 val2 ID name val1 val2
1 aaa01 10 100 NULL NULL NULL NULL
2 bbb01 20 200 NULL NULL NULL NULL
3 aaa02 30 300 3 ccc03 10 100
4 bbb02 40 400 4 ddd03 20 200

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