Question

I have a small SQL based challenge that i’m trying to solve to better my knowledge of Dynamic SQL.

My requirements are as follows.

I created a table that looks as follows:

CREATE TABLE Prison_Doors
(
 DoorNum INT IDENTITY(1,1) PRIMARY KEY,
 DoorOpen BIT,
 DoorClosed BIT,
 Trips INT
 )
 GO

I need to Create a Dynamic SQL Proc to insert 50 Door numbers and assign them as closed.

Expected result of proc:

|DoorNum|DoorOpen|DoorClosed|Trips|
|-------|--------|----------|-----|
|   1   |    0   |     1    |null |
|-------|--------|----------|-----|
|---------All the way to 50-------|
|-------|--------|----------|-----|
|   50  |    0   |     1    |null |

This is what I have written but it is not inserting:

BEGIN
DECLARE @SQL VARCHAR(8000)
DECLARE @Index INT
SET @Index=1


WHILE (@Index<=50)
BEGIN
    SET @SQL= 'INSERT INTO Prison_Doors(DoorNum,DoorOpen,DoorClosed)
                    VALUES('+CAST(@Index AS VARCHAR)+',0,1),'
    SET @Index=@Index+1
END

SET @SQL = SUBSTRING(@SQL, 1, LEN(@SQL)-1)
EXEC(@SQL)

END

I would like to know what I am doing wrong.

after all of this is done I then need to run another loop to start at door one and change every second door to open and change trips to one and then increment to every 3 doors to open and trips becomes 2 and this incrementation continues until all doors are open which will then select the number of trips that it took.

I hope somebody can assist me with this as I am new to Dynamic SQL and I just need some guidance and not the complete solution.

Help is much appreciated 🙂

Answer 1

The error you got is because you are trying to insert into an identity column DoorNumber:

DoorNum INT IDENTITY(1,1) PRIMARY KEY,

Remove that columns from the column list, instead of:

INSERT INTO Prison_Doors(DoorNum,DoorOpen,DoorClosed)

remove that column DoorNum:

INSERT INTO Prison_Doors(DoorOpen,DoorClosed)
...

---

However, there is no need for dynamic SQL to do that, you can do this using an anchor table like this:

 WITH temp 
 AS
 (
   SELECT n
   FROM (VALUES(1), (2), (3), (4)) temp(n)
 ), nums
 AS
 (
   SELECT ROW_NUMBER() OVER(ORDER BY (SELECT 1)) AS n
   FROM temp t1, temp t2, temp t3
 )
 INSERT INTO Prison_Doors(DoorOpen, DoorClosed)
 SELECT 0 AS DoorOpen, 1 AS DoorClosed
 FROM nums
 WHERE n <= 50;

Live Demo.

---

Update:

What my code does line by line?

Generating Sequence of numbers:

The first problem was generating a sequence of 50 numbers from 1 to 50, I used an anchor table with only four rows from 1 to 4 like this:

SELECT n
FROM (VALUES(1), (2), (3), (4)) temp(n);

This syntax using the VALUES is new to SQL-Server-2008, it is called Row Value Constructor. After the VALUES, you assign an alias of the table and the target columns in parentheses like temp(n).

For old versions you have to use something like :

SELECT n
FROM 
(
   SELECT 1 AS n
   UNION ALL 
   SELECT 2
   UNION ALL
   SELECT 3
   UNION ALL 
   SELECT 4
) AS temp;

This will give you only 4 rows, but we need to generate 50. Thats why I CROSS JOIN this table three time with itself using:

FROM temp t1, temp t2, temp t3

It is the same as

FROM temp t1
CROSS JOIN temp t2
CROSS JOIN temp t3

This will multiply these rows 64 times, 4 rows³ = 64 rows:

1   1   1
1   2   1
1   3   1
1   4   1
2   1   1
2   2   1
2   3   1
2   4   1
....
3   1   4
3   2   4
3   3   4
3   4   4
4   1   4
4   2   4
4   3   4
4   4   4

---

The Use Of ROW_NUMBER() Function:

Then using the ROW_NUMBER() will give us a ranking number from 1 to 64 like this:

 WITH temp 
 AS
 (
   SELECT n
   FROM (VALUES(1), (2), (3), (4)) temp(n)
 )
 SELECT ROW_NUMBER() OVER(ORDER BY (SELECT 1)) AS n
 FROM temp t1, temp t2, temp t3;

Note that: ROW_NUMBER requires an ORDER BY clause, in or case it doesn’t matter the order, so I used (SELECT 1).

There is also another way for generating a sequence numbers with out the use of ROW_NUMBER, it also depends on the CROSS JOIN, with an anchor table like:

 WITH temp 
 AS
 (
   SELECT n
   FROM (
         VALUES(0), (1), (2), (3), (4), (5), (6), (7), (8), (9)
        ) temp(n)
 ), nums
 AS
 (
   SELECT t1.n * 100 + t2.n * 10 + t3.n + 1 AS n
   FROM temp t1, temp t2, temp t3
 )
 SELECT n
 FROM nums 
 ORDER BY n;

Another Way of Generating Sequence Of Numbers

---

Common Table Expressions:

The CTE is called common table expression, and it was introdeced in SQL Server 2005. It is one of the table expressions types that SQL Server supports. The other three are:

• Derived tables,
• Views, and
• Inline table-valued functions.

It has a lot of important advantages. One of them is let you create a virtual tables that you can reuse them later, like what I did:

 WITH temp 
 AS
 (
   SELECT n
   FROM (VALUES(1), (2), (3), (4)) temp(n)
 ), nums
 AS
 (
   SELECT ROW_NUMBER() OVER(ORDER BY (SELECT 1)) AS n
   FROM temp t1, temp t2, temp t3
 )
 ...

Here I defined two CTE’s temp then another one nums that ruse that temp so this OL, you can create multiple CTE’s, just put a semicolon, then a new one with the AS clause and two ().

---

Insert into one table from another table using INSERT INTO ... SELECT ...

Now, we have a virtual table nums having rows from 1 to 64, we need to insert the rows from 1 to 50.

For this, you can use the INSERT INTO ... SELECT ....

Note that the columns in the INSERT clause are optional, but If you do so, you have to put a value for each row, if not you will got an error, for example if you have four columns and you put only three values in the VALUES clause or in the SELECT clause, then you will got an error. This is not valid for the idenetityt columns which are defined with:

Identity(1,1)
         ^ ^
         | |
         | ------------------The seed
         The start

In this case you simply ignore that column in the columns list in the INSERT clause and it will have the identity value. There is an option that let you insert a value manually like in the @Raj’s answer.

Note that: In my answer, I am not inserting the sequence numbers in to that column instead, inserting the values 50 times. But the actual numbers are generating automatically because of the Identity column:

 ...
 INSERT INTO Prison_Doors(DoorOpen, DoorClosed)
 SELECT 0 AS DoorOpen, 1 AS DoorClosed
 FROM nums;
 WHERE n <= 50;