SELECT Statement The following list provides additional information about other  SELECT  clauses: SELECT CONCAT(last_name,’, ’,first_name) AS full_name FROM mytable ORDER BY full_name; SELECT CONCAT(last_name,’, ’,first_name) full_name FROM mytable ORDER BY full_name; SELECT columna columnb FROM mytable; However, because the  AS  is optional, a subtle problem can occur if you forget the comma between two  select_expr  expressions: MySQL interprets the second as an alias name. For example, in the following statement,  columnb  is treated as an alias name: The  AS  keyword is optional when aliasing a  select_expr  with an identifier. The preceding example could have been written like this: For this reason, it is good practice to be in the habit of using  AS  explicitly when specifying column aliases. It is not permissible to refer to a column alias in a  WHERE  clause, because the column value might not yet be determined when the  WHERE  clause is executed. See Section B.3.4.4, “Problems with Column Aliases” . tbl_name  [[AS]  alias ] [ index_hint ] The use of index hints provides the optimizer with information about how to choose indexes during query processing. For a description of the syntax for specifying these hints, see Section 8.9.4, “Index Hints” . You can use  SET max_seeks_for_key= value  as an alternative way to force MySQL to prefer key scans instead of table scans. See Section 5.1.8, “Server System Variables” . SELECT college, region, seed FROM tournament ORDER BY region, seed; • Columns selected for output can be referred to in  ORDER BY  and  GROUP BY  clauses using column names, column aliases, or column positions. Column positions are integers and begin with 1: • A table reference can be aliased using  tbl_name  AS  alias_name  or  tbl_name alias_name . •You can refer to a table within the default database as  tbl_name , or as  db_name . tbl_name  to specify a database explicitly. You can refer to a column as  col_name ,  tbl_name . col_name , or db_name . tbl_name . col_name . You need not specify a  tbl_name  or  db_name . tbl_name  prefix for a column reference unless the reference would be ambiguous. See Section 9.2.2, “Identifier Qualifiers” , for examples of ambiguity that require the more explicit column reference forms. • The  FROM  table_references  clause indicates the table or tables from which to retrieve rows. If you name more than one table, you are performing a join. For information on join syntax, see Section 13.2.10.2, “JOIN Clause” . For each table specified, you can optionally specify an alias. • A  select_expr  can be given an alias using  AS  alias_name . The alias is used as the expression’s column name and can be used in  GROUP BY ,  ORDER BY , or  HAVING  clauses. For example: These statements are equivalent: SELECT t1.name, t2.salary FROM employee AS t1, info AS t2 WHERE t1.name = t2.name; SELECT t1.name, t2.salary FROM employee t1, info t2 WHERE t1.name = t2.name; 2663