sql - Equals(=) vs. LIKE for date data type

Question

First, I am aware that this question has been posted generally Equals(=) vs. LIKE. Here, I query about date type data on ORACLE database, I found the following, when I write select statment in this way:

SELECT ACCOUNT.ACCOUNT_ID, ACCOUNT.LAST_TRANSACTION_DATE
FROM ACCOUNT
WHERE ACCOUNT.LAST_TRANSACTION_DATE LIKE '30-JUL-07';

I get all rows I'm looking for. but when I use the sign equal = instead :

SELECT ACCOUNT.ACCOUNT_ID, ACCOUNT.LAST_TRANSACTION_DATE
FROM ACCOUNT
WHERE ACCOUNT.LAST_TRANSACTION_DATE = '30-JUL-07';

I get nothing even though nothing is different except the equal sign. Can I find any explanation for this please ?

Answer 1

Assuming LAST_TRANSACTION_DATE is a DATE column (or TIMESTAMP) then both version are very bad practice.

In both cases the DATE column will implicitly be converted to a character literal based on the current NLS settings. That means with different clients you will get different results.

When using date literals always use to_date() with(!) a format mask or use an ANSI date literal. That way you compare dates with dates not strings with strings. So for the equal comparison you should use:

LAST_TRANSACTION_DATE = to_date('30-JUL-07', 'dd-mon-yy')

Note that using 'MON' can still lead to errors with different NLS settings ('DEC' vs. 'DEZ' or 'MAR' vs. 'MRZ'). It is much less error prone using month numbers (and four digit years):

LAST_TRANSACTION_DATE = to_date('30-07-2007', 'dd-mm-yyyy')

or using an ANSI date literal

LAST_TRANSACTION_DATE = DATE '2007-07-30'

Now the reason why the above query is very likely to return nothing is that in Oracle DATE columns include the time as well. The above date literals implicitly contain the time 00:00. If the time in the table is different (e.g. 19:54) then of course the dates are not equal.

To workaround this problem you have different options:

1. use trunc() on the table column to "normalize" the time to 00:00 trunc(LAST_TRANSACTION_DATE) = DATE '2007-07-30 this will however prevent the usage of an index defined on LAST_TRANSACTION_DATE
2. use between
LAST_TRANSACTION_DATE between to_date('2007-07-30 00:00:00', 'yyyy-mm-dd hh24:mi:ss') and to_date('2007-07-30 23:59:59', 'yyyy-mm-dd hh24:mi:ss')

The performance problem of the first solution could be worked around by creating an index on trunc(LAST_TRANSACTION_DATE) which could be used by that expression. But the expression LAST_TRANSACTION_DATE = '30-JUL-07' prevents an index usage as well because internally it's processed as to_char(LAST_TRANSACTION_DATE) = '30-JUL-07'

The important things to remember:

1. Never, ever rely on implicit data type conversion. It will give you problems at some point. Always compare the correct data types
2. Oracle DATE columns always contain a time which is part of the comparison rules.