DQL

DQL is used for reading or querying data or table metadata. SQL statements involving SELECT and SHOW fall under this category.

SELECT is a very commonly used SQL syntax to read table data. It takes the following parameters:

• ALL: Doesn't eliminate duplicate rows.
• DISTINCT: Eliminates duplicate rows.
• DISTINCT ON ( a_expression): Eliminates duplicate rows based on a scalar expression.
• target_element: A scalar expression to determine a column in each result row, or to retrieve all columns in case of an asterisk (*).
• table_expression: A table expression from which the data has to be retrieved.
• AS OF SYSTEM TIME  timestamp: Retrieves data as it existed at the time of this timestamp, where timestamp refers to a specific time in the past. This can return historical data, which can be stale.
• WHERE a_expression: Only retrieves rows that return TRUE for a_expression.
• GROUP BY a_expression: Groups results on one or more columns.
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