SELECT Statement

The SELECT statement can be executed to retrieve data from a table or view. The result is stored in a result table, which is called a result set.

Serving as an overlaid filter for a database table, the SELECT statement filters required data from the table using SQL keywords.

Syntax

SELECT 
{ * | [column, ...] }
[ FROM from_item [, ...] ];

Parameter Description

  • SELECT list

    Specifies the name of a column in the table to be queried. The value can be a part of the column name or all of the column names. The wildcard (*) is used to represent all column names.

    Column names can be expressed in the following formats:

    • Manually input column names which are separated by commas (,)
    • Columns computed in the FROM clause
  • FROM clause

    Specifies one or more source tables for the SELECT statement. The FROM clause involves multiple elements. The common elements are as follows:

    • table_name

      Specifies the name of a table or view. The schema name can be added before the table name or view name, for example, schema_name.table_name. The table name is the most common element.

    • subquery

      Performs a subquery in the FROM clause. A temporary table is created to save subquery results. For details, see Subqueries.

    • alias

      Gives a temporary alias to a table to facilitate the reference by other queries. For details, see Aliases.

    • join_type

      There are five types. For details, see JOIN.

Examples

  • Read all data from the customer_t1 table.

    openGauss=# SELECT * FROM customer_t1;
    c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
    ---------------+---------------+--------------+-------------+--------
              3869 | hello         | Grace        |             |   1000
              3869 | hello         | Grace        |             |   1000
              3869 |               | Grace        |             |
              3869 | hello         |              |             |
              3869 | hello         |              |             |
                   |               |              |             |
              6985 | maps          | Joes         |             |   2200
              9976 | world         | James        |             |   5000
              4421 | Admin         | Local        |             |   3000
    (9 rows)
    
  • Read the c_customer_sk and c_customer_id columns from the customer_t1 table.

    openGauss=# SELECT c_customer_sk,c_customer_id FROM customer_t1;
    c_customer_sk | c_customer_id
    ---------------+---------------
              3869 | hello
              3869 | hello
              3869 |
              3869 | hello
              3869 | hello
                   |
              6985 | maps
              9976 | world
              4421 | Admin
    (9 rows)
    
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    openGauss 2024-04-25 00:46:51
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