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• A SELECT statement can be part of a union query or a subquery of another query.
• table_reference indicates the input to the query. It can be a regular table, a view, a join construct or a subquery.
• Table names and column names are case insensitive.
  • In Hive 0.12 and earlier, only alphanumeric and underscore characters are allowed in table and column names.
  • In Hive 0.13 and later, column names can contain any Unicode character (see HIVE-6013). Any column name that is specified within backticks (`) is treated literally. Within a backtick string, use double backticks (``) to represent a backtick character.
  • To revert to pre-0.13.0 behavior and restrict column names to alphanumeric and underscore characters, set the configuration property hive.support.quoted.identifiers to none. In this configuration, backticked names are interpreted as regular expressions. For details, see Supporting Quoted Identifiers in Column Names (attached to HIVE-6013). Also see REGEX Column Specification below.
• Simple query. For example, the following query retrieves all columns and all rows from table t1.

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A SELECT statement can take regex-based column specification in Hive releases prior to 0.13.0, or in 0.13.0 and later releases if the configuration property hive.support.quoted.identifiers is set to none. 

• We use java Java regex syntax. Try http://www.fileformat.info/tool/regex.htm for testing purposes.
• The following query select selects all columns except ds and hr.

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