达梦没有date_trunc函数，有trunc函数

可以根据语句修改下

CREATE OR REPLACE FUNCTION date_trunc(input_timestamp TIMESTAMP, trunc_unit VARCHAR2)
RETURN TIMESTAMP IS
    result_timestamp TIMESTAMP;
BEGIN
    CASE trunc_unit
        WHEN 'week' THEN
            SELECT TRUNC(input_timestamp, 'IW') INTO result_timestamp FROM DUAL;
        WHEN 'quarter' THEN
            SELECT TRUNC(input_timestamp, 'MONTH') INTO result_timestamp FROM DUAL;
        WHEN 'year' THEN
            SELECT TRUNC(input_timestamp, 'YEAR') INTO result_timestamp FROM DUAL;
        ELSE
            RAISE_APPLICATION_ERROR(-20001, 'Invalid truncation unit. Please specify WEEK, MONTH, or YEAR.');
    END CASE;
    
    RETURN result_timestamp;
END;
/
SELECT date_trunc(SYSDATE, 'week') AS truncated_week,
       date_trunc(SYSDATE, 'quarter') AS truncated_month,
       date_trunc(SYSDATE, 'year') AS truncated_year
FROM DUAL;