Different Ways to Alias a Column

Different ways to alias a column

I'd prefer the first one, since the second one is not portable -

select  EmployeeName = empName from employees

is either a syntax error (at least in SQLite and Oracle), or it might not give you what you expect (comparing two columns EmployeeName and empName and returning the comparison result as a boolean/integer), whereas

select  empName EmployeeName from employees

is the same as

  select  empName as EmployeeName from employees

which is my preferred variant.

How to reuse column alias in another column within same select statement?

In SQL Server, you can use APPLY to define column aliases in the FROM clause -- a good place, in my opinion:

select pd.id v.value_1,
       (Case when value_1 in ('test1', 'test2') Then '0'
             else value_1
        End) as value_2
from pd cross apply
     (values ( case . . . )
     ) v(value_1);

Notes:

• Don't use identifier names that need to be escaped, hence value_1 instead of value 1.
• value_1 appears to be string. The case expression for value_2 should return a string, so '0' instead of 0.
• I define table aliases using as after the expression. This is the SQL standard, but there is nothing per se wrong with using =, which is SQL Server specific syntax.

Dynamic column alias from another column value in SELECT

It seems that there is no way to create the column alias dynamically without knowing the values since the beginning. As many commented the only way to achieve this kind of "table re-mapping" is to use the crosstab function.

Crosstab function summary

This function takes 2 arguments:

• The first one is a SQL statement that must return 3 columns:
  
  • The first column contains the values identifying each instance and that must be grouped in order to get the final result.
  • The second column contains the values that are used as categories in the final pivot table: each value will create a separate column.
  • The third column contains the values used to compile the new columns formed: for each category this column has the value of the instance that had the category value in the original table.
• The second argument is not mandatory and is a SQL statement that returns the distinct values the function should use as categories.

Example

In the example above we must pass a query to crosstab that:

• Returns as the first column the identifier of each final instance (in this case id)
• As second column the values used as categories (all values in key)
• As third column the values used to fill the categories (all values in value)

So the final query should be:

select * from crosstab(
    'select "id", "key", "value" from testTable order by 1, 2;',
    'select distinct "key" from testTable order by 1;'
) as result ("id" int8, "a" text, "b" text);

Since the crosstab function requires a column definition for the final pivot table, there is no way to determine the column alias dynamically.

Dynamically infer column names with client

A possible way to do that, with a PostgreSQL client, is to launch the second query we passed as argument to crosstab in order to retrieve the final columns and then infer the final crosstab query.

As an example, with pseudo-javascript:

const client;

const aliases = client.query(`select distinct "key" from testTable order by 1;`);

const finalTable = client.query(`select * from crosstab(
    'select "id", "key", "value" from testTable order by 1, 2;',
    'select distinct "key" from testTable order by 1;'
) as result ("id" int8, ${aliases.map(v => v + ' data_type').join(',')});`)

Useful articles

https://learnsql.com/blog/creating-pivot-tables-in-postgresql-using-the-crosstab-function/

How to select values with a column alias?

I think you are looking for this:

SELECT *
FROM (VALUES (1), (2), (3)) AS tbl(col)
WHERE tbl.col = 1;

Note that when you escape an identifier (using "tbl.col"), then that is one name that has a period in it. Not two names.

EDIT:

I would have expected the above to work, but it doesn't in SQLite. One alternative is to use a CTE:

with tbl(col) as (
      VALUES (1), (2), (3)
     )
SELECT *
FROM tbl
where tbl.col = 1

I want to change column alias dynamically as current_date() in bigquery

Consider below approach

execute immediate ('''
select * except(row_id) from (
select metric, dN, current_date - N as col, row_id
from ( 
  select "inspection" as metric, d1, d2, d3, d4, d5, d6, to_json_string(t) as row_id 
  from your_table t
), unnest([
  if(d1 is null,"insp_not_done",d1), 
  if(d2 is null,"insp_not_done",d2), 
  if(d3 is null,"insp_not_done",d3), 
  if(d4 is null,"insp_not_done",d4), 
  if(d5 is null,"insp_not_done",d5), 
  if(d6 is null,"insp_not_done",d6)
]) dN with offset as N
)
pivot (min(Dn) for col in (''' ||
(select string_agg("'" || date || "'", ',' order by offset desc)
from unnest(generate_date_array(current_date - 5, current_date)) date with offset) || "))"
)

if apply to dummy data like in below example

create temp table your_table as (
  select '1' d1, null d2, '3' d3, '4' d4, null d5, '6' d6 union all
  select '21' d1, '22' d2, null d3, '24' d4, '25' d5, '26' d6   
);        

output is

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