如何在SQL中别名特定值？

Question

I will present a question about 'aliasing' values from a column. I will use days of the week as an intuitive example to get my question across, but I am not asking for datetime conversions.

Suppose I have the following SQL script:

SELECT DaysOfWeek 
FROM [databasename].[dbo].[tablename]

Now, the column DaysOfWeek will return string values of the days' names, ie "Monday," "Tuesday," and so forth.

What if I wanted the query to return the integer 1 for 'Monday', 2 for 'Tuesday', and so forth? I would want to assign a particular value to each of the week's days in the SELECT statement, but I'm not sure how to go about doing that.

I'm relatively new to SQL, so I just thought I'd ask for an intuitive method to perform such a task.

Edited to add: I'm only using days of the week and their respective integer representation as an easy example; my task does not involve days of the week, but rather employee code numbers and corresponding titles.

Answer 1

You can do this using case :

SELECT (CASE DaysOfWeek 
             WHEN 'Monday' THEN 1
             WHEN 'Tuesday' THEN 2
             . . .
        END)

Under most circumstances, it is unnecessary to store the day of the week like this. You can readily use a function, datepart() or datename() , to extract the day of the week from a date/time value.

If the column is in a table, and not part of a date, then you might want to include the above logic as a computed column:

alter table t add DayOfWeekNumber as (case DaysOfWeek when 'Monday' then 1 . . .);

Answer 2

If you wanted to define your own corresponding value for another value, the best way is to use a table, and join that table.

For example:

create table dbo.EmployeeTitle (
    id int not null identity(1,1) primary key
  , title varchar(32)
  );

create table dbo.Employee (
    id int not null identity(1,1) primary key
  , name nvarchar(128)
  , title_id int references dbo.EmployeeTitle(id)
  );

insert into dbo.EmployeeTitle values ('Big boss');
insert into dbo.Employee values ('daOnlyBG',1);

select e.*, et.title
from dbo.Employee e
  inner join dbo.EmployeeTitle et
    on e.title_id = et.id

rextester demo: http://rextester.com/FXIM78632

returns:

+----+----------+----------+----------+
| id |   name   | title_id |  title   |
+----+----------+----------+----------+
|  1 | daOnlyBG |        1 | Big boss |
+----+----------+----------+----------+

Answer 3

Use CASE, here you have the definition and one example :

select
    CASE 
          WHEN(DaysOfWeek="Monday") THEN 1
          WHEN(DaysOfWeek="Thusday") THEN 2
          ....
    ELSE  -1 
from table

Hope this help!

Answer 4

The easiest way I can think of is to have a table variable or CTE; create your lookup as rows and join to it. Something like this:

with cte as (
  select 1 as emp_code, 'value1' as emp_title
  union
  select 2 as emp_code, 'value2' as emp_title
  union
  select 3 as emp_code, 'value3' as emp_title
)

select cte.emp_code, tableName.*
from tableName
inner join cte
on cte.emp_title = tableName.some_column