c# print barcode labels Inside Microsoft SQL Server 2008: T-SQL Querying in Visual C#

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Inside Microsoft SQL Server 2008: T-SQL Querying
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LISTING 6-1 Creating and populating the table T1
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USE tempdb; GO IF OBJECT_ID('dbo.T1') IS NOT NULL DROP TABLE dbo.T1; GO CREATE TABLE dbo.T1 ( keycol INT NOT NULL PRIMARY KEY CHECK(keycol > 0), datacol VARCHAR(10) NOT NULL ); INSERT INTO dbo.T1(keycol, datacol) VALUES (3, 'a'), (4, 'b'), (6, 'c'), (7, 'd');
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Notice that keycol must be positive. Your task is to write a query that returns the lowest missing key, assuming that key values start at 1. For example, the table is currently populated with the keys 3, 4, 6, and 7, so your query should return the value 1. If you insert two more rows, with the keys 1 and 2, your query should return 5. Here s a suggested CASE expression (incomplete) that I used in my solution:
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SELECT CASE WHEN NOT EXISTS(SELECT * FROM dbo.T1 WHERE keycol = 1) THEN 1 ELSE (...subquery returning minimum missing value...) END;
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If 1 doesn t exist in the table, the CASE expression returns 1; otherwise, it returns the result of a subquery returning the minimum missing value. Here s the subquery that I used to return the minimum missing value:
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SELECT MIN(A.keycol) + 1 as missing FROM dbo.T1 AS A WHERE NOT EXISTS (SELECT * FROM dbo.T1 AS B WHERE B.keycol = A.keycol + 1);
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The NOT EXISTS predicate returns TRUE only for values in T1 that are right before a gap (4 and 7 in our case). A value is right before a gap if the value plus one does not exist in the same table. The outer T1 table has the alias A, and the inner T1 table has the alias B. You could use the expression B.keycol 1 = A.keycol in the subquery s lter, although it might be a bit confusing to use such an expression when looking for a value in B that is greater than the value in A by one. If you think about it, for B.keycol to be greater than A.keycol by one, B.keycol minus one must be equal to A.keycol. If this logic confuses you, you can use B.keycol = A.keycol + 1 instead, as I did. When all points before gaps are isolated, the outer
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Subqueries, Table Expressions, and Ranking Functions
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query returns the minimum plus one, which is the rst missing value in the rst gap. Make a mental note of the technique to identify a point before a gap it s a very handy fundamental technique. Now you can incorporate the query returning the minimum missing value in the CASE expression:
SELECT CASE WHEN NOT EXISTS(SELECT * FROM dbo.T1 WHERE keycol = 1) THEN 1 ELSE (SELECT MIN(A.keycol) + 1 FROM dbo.T1 AS A WHERE NOT EXISTS (SELECT * FROM dbo.T1 AS B WHERE B.keycol = A.keycol + 1)) END;
If you run this query with the sample data inserted by Listing 6-1, you should get 1 as the result. If you then insert two more rows, with the keys 1 and 2 (as shown in the following code), and rerun the query, you should get 5 as the result.
INSERT INTO dbo.T1(keycol, datacol) VALUES(1, 'e'),(2, 'f');
Here is an example of how you might use the CASE expression for the minimum missing key in an INSERT . . . SELECT statement, perhaps in a scenario where you needed to reuse deleted keys:
INSERT INTO dbo.T1(keycol, datacol) SELECT CASE WHEN NOT EXISTS(SELECT * FROM dbo.T1 WHERE keycol = 1) THEN 1 ELSE (SELECT MIN(A.keycol) + 1 FROM dbo.T1 AS A WHERE NOT EXISTS (SELECT * FROM dbo.T1 AS B WHERE B.keycol = A.keycol + 1)) END, 'g';
Query the T1 table after running this INSERT:
SELECT * FROM dbo.T1;
Notice in the following output that the insert generated the key value 5, which was the minimum missing key:
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