SQL Server 2012 : Introducing Basic Query Flow - (0 row(s) affected)Columns, Stars, Aliases, and Expressions

The title of this section may read like a bad tabloid headline, but in all seriousness it means that the SQL SELECT statement returns columns in the order in which you list them in the SELECT statement. The column may be any expression or any column in the FROM clause.

Following the FROM clause and the WHERE clause, the next logical step in the query is the list of returned expressions.

The Star

The *, commonly called “star,” is a special wildcard that includes all columns in their table order. If the query pulls from multiple tables, the * includes all columns from every table. Alternatively, tablename.* includes only the columns from the named table. Also, if you alias any tables in the FROM clause, you can return a complete column list of a specific example by using table alias.*.

Aliases

The name of the column in the underlying table becomes the name of the column in the result set. Optionally, you can change the column name using a column alias.

Expressions and constants have a blank column heading in the result set unless an alias is provided.

The AS keyword is optional, but just as with a table alias, using it is a good practice that improves the readability of the code and helps prevent errors.

To use an alias that's identical to a SQL Server keyword or that includes a space, enclose the alias in square brackets, single quotes, or double quotes. Although the square brackets are not technically required if the alias is the same as an object name (that is, table or column name), you an explicitly specify that the alias is not a keyword.

The following code demonstrates adding aliases to columns:

USE AdventureWorks;
SELECT 
    Name AS ProductName,
    ‘abc',
    SellStartDate + 365 OneYearSellStartDate
FROM Production.Product;

Result:

ProductName           OneYearSaleStartDate
-------------------------- ---- ------------------------
Adjustable Race      abc  2003-06-01 00:00:00.000
Bearing Ball        abc  2003-06-01 00:00:00.000
BB Ball Bearing      abc  2003-06-01 00:00:00.000
. . .

The first column's name is changed from Name to ProductName by means of an alias. The second column is an expression without an alias, so it has no column name. A better practice is to name expression columns using an alias, as demonstrated in the third column.

Accidental aliases are a common source of errors. Take a careful look at the next query:

USE AdventureWorks;
SELECT 
    Name 
    ‘abc',
    SellStartDate + 365 OneYearSellStartDate
FROM Production.Product;

Result:

abc            OneYearSaleStartDate
------------------------- -------------------------
Adjustable Race      2003-06-01 00:00:00.000
Bearing Ball        2003-06-01 00:00:00.000
BB Ball Bearing      2003-06-01 00:00:00.000.. .

The second column isn't abc as in the previous query. Instead, because of a missing comma, the ‘abc' in the query became an accidental alias for the first column.

Qualified Columns

A common problem with queries is that column names are duplicated in multiple tables. Including the column in the select list by column names alone can cause an “ambiguous column name” error. Basically, SQL Server complains that it doesn't know which column you refer to. Even if they contain the same exact data, SQL Server must know which column to select.

CREATE TABLE t1 (col1 INT);
CREATE TABLE t2 (col1 INT);

SELECT col1
 FROM t1
  CROSS JOIN t2;

Result:

Msg 209, Level 16, State 1, Line 2
Ambiguous column name ‘col1'.

The solution, of course, is to qualify the column name by including the table:

SELECT t1.col1
 FROM t1
  CROSS JOIN t2;