Question

I need to build a primary key consisting of 3 sections.

• The first section is 3 letters long
  and must be an a followed by two
  digits eg “a00”
• The second section is a 3 digit
  unique identifier INCLUDING LEADING
  ZEROES, example “003”
• The third section is the year section
  of the date, eg “2008”

This is all separated with dashes, so a complete key would look like “a00-014-2008”

My solution is to hold three separate columns with the data for each, unformatted, then add the clustered primary key constraint and format it when I need to display it in a view. Is this a good way of going about it? I think there might be a way to make the whole key on update using a trigger but I don’t know how. Also how would I concatenate and display it in a view?

Though not a homework question, this will help me academically.

Answer 1

CREATE TABLE mytable (
      first TINYINT NOT NULL CHECK(first BETWEEN 0 AND 99),
      second INT NOT NULL CHECK (second BETWEEN 0 AND 999),
      third INT NOT NULL,
      CONSTRAINT pk_mytable_123 PRIMARY KEY (first, second, third)
      )
GO
CREATE VIEW v_mytable AS
SELECT  'a' + RIGHT('00' + CAST(first AS VARCHAR), 2) AS first,
        RIGHT('0000' + CAST(second AS VARCHAR), 4) AS second,
        third
FROM    mytable
GO

INSERT
INTO    mytable
VALUES  (1, 1, 2008)
GO

SELECT  *
FROM    v_mytable
GO

first  second  third
----   ----    ----
a01    0001    2008

This primary key will hold only INT values, all formatting will be done in the view.

Note that to achive best results in terms of performance, you better create computed columns instead of the view, and create an additional UNIQUE index in these columns:

CREATE TABLE mytable (
      _first TINYINT NOT NULL CHECK(_first BETWEEN 0 AND 99),
      _second INT NOT NULL CHECK (_second BETWEEN 0 AND 999),
      third INT NOT NULL,
      first AS 'a' + RIGHT('00' + CAST(_first AS VARCHAR), 2),
      second AS RIGHT('0000' + CAST(_second AS VARCHAR), 4)
      CONSTRAINT pk_mytable_123 PRIMARY KEY (_first, _second, third)
      )

CREATE UNIQUE INDEX ux_mytable_123 ON mytable (first, second, third)