t-sql insert statement where primary key is a decimal - sql-server

Question

2 years ago I created a table that has 22 rows. Each row is a step/page in filing an application for hire. I realized back then I would most likely be asked to insert steps as the business grew. I was right. I need to insert a new step between step 21 & 22. So I want to create a new row in that table with stepId = 21.5. But the insert statement fails.
INSERT INTO frznStep (
stepId
,myField1
,myField2
,myField3
)
VALUES (
21.5
,'xxx'
,'yyy'
,'zzz'
)
the error msg is:
Violation of PRIMARY KEY constraint 'PK_frznStep'. Cannot insert
duplicate key in object 'dbo.frznStep'. The duplicate key value is
(22).

Answer 1

I suspect when you script out the table, you'll see that the precision of your decimal column is 0, so something like stepId decimal(9,0)
If you have a non-zero value for the decimal precision, the following repro works
USE tempdb
DROP TABLE IF EXISTS #frznStep;
CREATE TABLE #frznStep
(
stepId decimal(9, 1) NOT NULL
, myField1 varchar(255) NOT NULL
, myField2 varchar(255) NOT NULL
, myField3 varchar(255) NOT NULL
, CONSTRAINT PK_frznStep PRIMARY KEY (stepId)
);
insert into #frznStep (stepId, myField1, myField2, myField3) values (21, 'www', 'yyy', 'zzz');
insert into #frznStep (stepId, myField1, myField2, myField3) values (22, 'yyy', 'yyy', 'zzz');
insert into #frznStep (stepId, myField1, myField2, myField3) values (21.5, 'xxx', 'yyy', 'zzz');
GO
When you use a 0 scale, you'll get 21 and 22 into the table but 21.5 will be implicitly converted to decimal(x,0) which then violates the primary key constraint.
-- Redeclare as 0 precision
DROP TABLE IF EXISTS #frznStep;
CREATE TABLE #frznStep
(
stepId decimal(9, 0) NOT NULL
, myField1 varchar(255) NOT NULL
, myField2 varchar(255) NOT NULL
, myField3 varchar(255) NOT NULL
, CONSTRAINT PK_frznStep PRIMARY KEY (stepId)
);
insert into #frznStep (stepId, myField1, myField2, myField3) values (21, 'www', 'yyy', 'zzz');
insert into #frznStep (stepId, myField1, myField2, myField3) values (22, 'yyy', 'yyy', 'zzz');
--Msg 2627, Level 14, State 1, Line 36
--Violation of PRIMARY KEY constraint 'PK_frznStep'. Cannot insert duplicate key in object 'dbo.#frznStep'. The duplicate key value is (22).
--The statement has been terminated.
insert into #frznStep (stepId, myField1, myField2, myField3) values (21.5, 'xxx', 'yyy', 'zzz');
You options are either to change your data type to include the scale (which will require dropping and recreating the primary key as the column is part of it) Or scale everything up by a factor of 10 and then you can insert into the 215 nicely between 210 and 220. (A "trick" I learned the hard way programming Apple Basic ages ago)

Answer 2

The first intuitive answer off the bat would be to convert your primary key to a numeric type, such as decimal.
However, is there really a reason to think of the step as 21.5? Or are you just trying to fit it between 21 and 22? I say this because the more ideal situation would be to have a primary key that simply serves as an identity. Then have a separate column that identifies the step number. That way, instead of having the step be 21.5, you'll simply have it be step 22, and then you can change step 22 to step 23.
alter table frznStep add column stepOrd int null;
update frznStep set stepOrd = stepId;
update frznStep set stepOrd = 23 where stepOrd = 22;
insert frznStep (stepId, stepOrd, ...) values (100, 22, ...);
You could also convert stepId to autoincrement. Though I believe you'll have to drop the table and recreate it in that case.

Answer 3

You’re getting the error because your id column is effectively integer and your attempted insert value is being rounded to integer, thus colliding with an existing key value.
Rather than using the id column as both unique identifier and step order, which is a design flaw (overloading a column), specify the steps as a chain, like a linked list, by introducing a column, perhaps called nextStepId, that stores the id of the next step to run.
This would separate the concerns of primary key being the row identifier and step order, giving control of step order independent of id values being any particular value relative to each other.