选择带有通配符的列？

Question

是否可以在 Teradata 中选择带有通配符的列？

喜欢：

Select ABC_* FROM TABLE; -> 结果所有列都以 ABC 名称开头。

Answer 1

虽然我同意每个人的看法，这是一个奇怪的要求，但并非完全没有听说过。不幸的是，除了一些脚本之外，没有内置的 SQL 方法可以做到这一点。

这里的基本要点是，您不能通过 SQL 动态引用数据库对象（表、视图、过程、宏、函数、数据库、列等）。相反，您必须编写生成该 SQL 的脚本，然后执行它，然后将结果返回给您。

我们可以通过使用存储过程直接在我们的数据库中完成所有这些操作。

这是一个适用于 Teradata 的快速示例（在您将我们的 yourdatabase 和 yourtable 替换为您的实际数据库和表之后）：

CREATE PROCEDURE return_tablecolumns_with_ABC()

/*
 * We have to tell teradata that this will return
 * a single result set
 */
DYNAMIC RESULT SETS 1
BEGIN   

    /*
     * We need three variables here
     * 1. A varchar to hold our list of columns from dbc.columnsV
     * 2. A varchar to hold the dynamically generated SQL string
     * 3. A cursor to hold the result set of the SQL string
     */
    DECLARE column_list VARCHAR(1000);    
    DECLARE my_sql VARCHAR(500);    
    DECLARE my_cursor CURSOR WITH RETURN ONLY FOR my_statement;

    /*
     * First we query dbc.columsV for a list of columns 
     *  that match your criteria (exists in your table and
     *  and starts with ABC_)
     */
    SELECT TRIM(TRAILING ',' FROM (XMLAGG(trim(ColumnName) || ',' ORDER BY ColumnName) (VARCHAR(1000)))) INTO column_list
    FROM "DBC"."ColumnsV" 
    WHERE DatabaseName = 'yourdatabase' 
        AND TableName = 'yourtable'
        AND columnName LIKE 'ABC_%';

    /* 
     * Now we build our dynamically generated SQL string
     * This could use some polish as it will fail if your
     * columns contain spaces or anything that requires 
     * their names be encapsulated with double quotes...
     */
    SET my_sql = 'Select ' || column_list || ' FROM yourdatabase.yourtable;';

    /*
     * Now we prepare our statement from the dynamically
     * generated SQL string
     */
    PREPARE my_statement FROM my_sql;

    /*
     * And finally we open the Cursor we declared for
     * the statement we just prepared. 
     * We leave the cursor open so it will be returned
     * as a result set when this procedure is called.
     */
    OPEN my_cursor;

END;

您现在可以调用它来获取结果：

CALL return_tablecolumns_with_ABC();