从 Oracle 数据库中的年份获取月份和日期

Question

I am trying to get the date and month from Year entered by user.

Query I have used is:

SELECT trans_date FROM transactions WHERE (SUBSTR(trans_date,4,4) = 'June' AND TO_NUMBER(SUBSTR(trans_date,9,4)) = 2010); 

This is not working but if I do the same with DUAL table it works.

SELECT SUBSTR('22-June-2018', 4, 4) FROM DUAL;

What is the problem with it ?

Answer 1

Have you considered something like to_char() ?

SELECT trans_date
FROM transactions
WHERE TO_CHAR(trans_date, 'YYYY-MM') = '2010-06';

Or better yet, proper date comparisons:

WHERE trans_date >= DATE '2010-06-01' AND
      trans_date < DATE '2010-07-01'

This has the advantage that the query can use an index on trans_date .

Answer 2

SELECT to_char(to_date(trans_date,'DD-Month-YYYY'),'DD') day,
       to_char(to_date(trans_date,'DD-Month-YYYY'),'Month') month
FROM transactions
WHERE to_char(to_date(trans_date,'DD-Month-YYYY'),'Month') = 'June' 
AND to_char(to_date(trans_date,'DD-Month-YYYY'),'YYYY') = '2010'); 

Answer 3

What is the problem with it ?

SUBSTR( string_value, start, length ) takes a string data type as the 1st argument. '22-June-2018' is not a DATE data type - it is a string data type (that just happens to contain characters that we recognise as a date); so using SUBSTR( '22-June-2018', 4, 4 ) works as expected.

trans_date is a DATE data type and not a string data type.

SUBSTR( trans_date, 4, 4 )

Involves an implicit conversion from a date to a string; so is effectively doing:

SUBSTR( TO_CHAR( trans_date ), 4, 4 )

Since, TO_CHAR( date_value, format_model ) needs to know how to format the DATE to a string, if you do not provide a second argument then it will use the NLS_DATE_FORMAT session parameter and the expression is effectively:

SUBSTR(
  TO_CHAR(
    trans_date
    ( SELECT value FROM nls_session_parameters WHERE parameter = 'NLS_DATE_FORMAT' )
  ),
  4,
  4
)

If the NLS_DATE_FORMAT session parameter does not match DD-Month-YYYY (or something equivalent where the 4-7th characters are month (which it would only be for 4-character month names) and 9th-12th characters are year (again, this would only be true for months with 4-character names) then your query will not work.

I am trying to get the date and month from Year entered by user.

Use EXTRACT :

SELECT trans_date
FROM   transactions
WHERE  EXTRACT( MONTH FROM trans_date ) = 6
AND    EXTRACT( YEAR  FROM trans_date ) = 2010

Use TO_CHAR( date_value, format_model ) :

SELECT trans_date
FROM   transactions
WHERE  TO_CHAR( trans_date, 'Month' ) = 'June'
AND    TO_CHAR( trans_date, 'year'  ) = '2010'

Use date literals:

SELECT trans_date
FROM   transactions
WHERE  trans_date >= DATE '2010-06-01'
AND    trans_date < ADD_MONTHS( DATE '2010-06-01', 1 );

Use TRUNC( date_value, 'MM' ) :

SELECT trans_date
FROM   transactions
WHERE  TRUNC( trans_date, 'MM' ) = DATE '2010-06-01';

Answer 4

你可以使用像

WHERE TO_CHAR(date_column_name,'MM-YYYY')='06-2010'