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The MySQL Worklog are design specifications for some of the items that may be considered for future development.
# WL#831: Implement INTERVAL types

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The MySQL Worklog are design specifications for some of the items that may be considered for future development.

Affects: Server-7.1
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Status: Assigned
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Priority: Medium

Implement the interval types required by ANSI SQL. (The description used to say "in MySQL 5.0", that's no longer true.)

An interval is a difference between any two datetime values. It contains fields similar to datetime fields. It is specified in ANSI SQL, it is supported by Oracle + Informix, it is partially supported (with a very limited syntax subset) by PostgreSQL, it is not supported by SQL Server + DB2. Syntax for data type definition ------------------------------- INTERVAL interval-qualifier Interval-qualifier is one of: YEAR e.g. '1999' YEAR TO MONTH e.g. '1999-01' MONTH e.g. '01' DAY e.g. '31' DAY TO HOUR e.g. '31 05' DAY TO MINUTE e.g. '31 05:59' DAY TO SECOND e.g. '31 05:59:07' HOUR e.g. '18' HOUR TO MINUTE e.g. '18:19' HOUR TO SECOND e.g. '18:19:20' MINUTE e.g. '20' MINUTE TO SECOND e.g. '20:21' SECOND e.g. '21' In the above, the elements (YEAR MONTH DAY etc.) are "fields", and when there is more than one field the term "multi-field" will be used. There are no intervals like "MONTH DAY" because it's hard to interpret the meaning of such a value, given that months have varying numbers of days. Therefore the year-month intervals (YEAR, YEAR TO MONTH, DAY) are incompatible with the day-second intervals (all the rest), for some operations that will be described later. Each field has a default precision (the number of digits it can contain). The first field of a multi-field interval, or any single-field interval, may have a higher precision. For example: INTERVAL DAY precision is default precision 2 (up to 99), but you may increase it to 4 by specifying INTERVAL DAY (4). For each field, here is the default precision and the maximum precision you could specify: Field Default Maximum specifiable ----- ------- ------------------- YEAR 4 6 MONTH 2 4 DAY 2 4 HOUR 2 4 MINUTE 2 4 SECOND 2 4 (Peter just made the maximum numbers up, if they cause trouble then say so.) There may be a (leading precision) on the first field (described later), and a (fractional seconds precision) on a SECOND field (described later). Any column or declared variable may have an interval data type. DAY, HOUR, INTERVAL, MINUTE, MONTH, SECOND, YEAR become new reserved words. Syntax for literal ------------------ The standard syntax is: INTERVAL '...' interval-type Within the ''s, the value should be a number or a series of numbers. Depending on interval-type, the separator between the numbers may be ' ' or ':' just as within a datetime literal. Leading zeros are optional. An interval may contain a sign, either inside or outside the ''s. For example: INTERVAL '+15' HOUR or INTERVAL -'17:18' HOUR TO MINUTE. It is smarter to put the symbol inside the ''s, because MySQL will allow replacing the '...' with a variable, as in SET @a='+15'; SELECT CURRENT_DATE + INTERVAL @a HOUR; MySQL Extension #1. Support syntax without lead word INTERVAL. For example: '05:06' HOUR TO MINUTE MySQL Extension #2. Allow removal of ''s when there is only one field. For example: INTERVAL 5 MINUTE MySQL Extension #3. Allow removal of interval-type when it's clear from the format. For example: INTERVAL '01:01:01' /* must be hour:minute:second */ INTERVAL '1999-04' /* must be year-month */ Literals which don't follow the prescribed format, or contain too-large integers, will cause errors or warnings depending on sql_mode=strict. Datetime/Datetime or Datetime/Interval arithmetic ------------------------------------------------- When date arithmetic takes place, the result is an interval. One may specify the interval-type so that it's clear what specific kind of interval. That is: (datetime - datetime) interval-type (datetime + interval) interval-type (datetime - interval) interval-type The effect is like "casting". For example: (DATE '1999-04-01' - DATE '1998-04-01') YEAR The casting is necessary in standard SQL, but MySQL already allows "date - date", returning days, and it's okay to preserve that extension. Interval/Interval arithmetic ---------------------------- It is also possible to add or subtract two intervals. The intervals must be of the same general type, a year-month interval can't go with a day-second interval. If the intervals are of the same general type, but have different fields, the result data type is an interval with all the fields that are in either of the original values. For example: INTERVAL '1' DAY + INTERVAL '1' YEAR is illegal. INTERVAL '1' DAY + INTERVAL '1' DAY results in INTERVAL '2' DAY. INTERVAL '1 5' DAY TO HOUR + INTERVAL '1' SECOND results in INTERVAL '1 5:0:1' DAY TO SECOND. One may specify the interval-type. For example: (INTERVAL '1' DAY - INTERVAL '1' DAY) DAY. Datetime/Interval/Interval arithmetic ------------------------------------- Since year-month intervals won't go with day-second intervals, there can be some unnecessary difficulty. Melton gives this example: (DATE '1999-12-01' + INTERVAL '01' MONTH) + INTERVAL '1' DAY is legal. DATE '1999-12-01' + (INTERVAL '01' MONTH + INTERVAL '1' DAY) is illegal. Therefore the DBMS must observe parentheses, and if there are no parentheses, must handle expressions in the standard order -- left to right. Interval/Integer arithmetic --------------------------- It is legal to multiple an interval by an integer. It is legal to divide an interval by an integer. For example: INTERVAL '2-1' YEAR TO MONTH * 2 The result should be INTERVAL '4-2' YEAR TO MONTH. Or is it? See next section. Normal Interpretation for Multi-Field Intervals ----------------------------------------------- Sometimes a multi-field interval can be expressed with everything in the smallest field ("'00 61' YEAR TO MONTH) or with some in the larger field ("'05 01' YEAR TO MONTH). The normal interpretation would be based on the fact that the user said YEAR TO MONTH rather than MONTH. So there's a desire to see the years, and '05 01' is right after a calculation. So "normalization" would work thus: If seconds > 59, carry to minutes. If minutes > 59, carry to hours. If hours > 23, carry to days. If months > 11, carry to years. Even if users deliberately enter '00 61' into an INTERVAL YEAR TO MONTH column, do not preserve it. Normalize it to '05 01'. Someday they'll thank you. Functions --------- The following function will work with interval values: EXTRACT (field FROM interval value) For example, EXTRACT (DAY FROM INTERVAL '01 02' DAY TO HOUR) returns 01. The following functions, although they are for datetime arithmetic, probably will not work with interval values. The expectation is that nobody cares. ADDDATE(), SUBDATE() ADDTIME(), SUBTIME() DATEDIFF(), TIMEDIFF() DATE_ADD, DATE_SUB() TIMESTAMPADD(), TIMESTAMPDIFF() DATE_FORMAT(), GET_FORMAT() FROM_DAYS MICROSECOND(), MINUTE(), HOUR(), DAY(), etc. PERIOD_ADD(), PERIOD_DIFF() Data type for connectors ------------------------ Properly, an INTERVAL value should be passed to a connector or mysql client as an INTERVAL. In fact MySQL will just pass it as a string. Coming the other way (from client to server), MySQL will accept either any string for an interval, provided it's formatted as described in earlier section 'Syntax for literal'. Fractional seconds ------------------ An interval may include fractional seconds, after the completion of WL#946 TIME/TIMESTAMP/DATETIME with fractional seconds Then an interval literal may include a non-integer SECONDS component. For example: INTERVAL 1.5 SECOND Then an interval data type definition may include (n) where n is an integer between 0 and 6. For example: INTERVAL SECOND (6) Monty's comment: "For the first implementation, we should not support [fractional seconds] precisions as we can't yet store this in the .frm file. (To be done in 5.1)" Storage ------- Monty says: "The simple types like 'interval year' can be stored as a tinyint The others should be stored as 2-4 byte ints, depending on circumstances For example year-to-month should be stored as 'number of months' and 2 bytes would probably sufficient for this." Peter says: "Every interval is really a DECIMAL(26). The digits of an interval are as follows: YEAR 6 digits MONTH 4 digits DAY 4 digits HOUR 4 digits MINUTE 4 digits SECOND 4 digits The main advantage is that you can convert easily to a number rather than an array of numbers. Arithmetic is tricky because a carry isn't always the same. Comparison is not tricky because MySQL forces numbers to be normal. Arithmetic ---------- As well as the type, we need to support the arithmetic. Currently, MySQL does allow what looks like an INTERVAL expression, but this is "pseudo-interval support", it ignores essentials. The following does not work in a standard way: CREATE TABLE F052B (S1 TIME) INSERT INTO F052B VALUES ('00:00:00') INSERT INTO F052B VALUES ('23:59:59') INSERT INTO F052B VALUES ('01:02:03') SELECT S1, S1 + INTERVAL '1' SECOND FROM F052B (Monty has explained that this occurs because MySQL will attempt to interpret the TIME as a DATE.) At the Budapest conference, Monty suggested that correct interval arithmetic is only priority = 50. So Peter believes that interval arithmetic has a lower priority than interval data type support. INFORMATION_SCHEMA ------------------ In all cases where there is currently a data type descriptor, there must be an indication of the interval-type and interval-precision. (These are not simply part of the data type name.) For example, INFORMATION_SCHEMA.COLUMNS requires two new columns: INTERVAL_TYPE VARCHAR() CHARACTER SET UTF8 ... here INTERVAL_TYPE is one of: 'YEAR', 'MONTH', 'DAY', 'HOUR', 'MINUTE', 'SECOND', 'YEAR TO MONTH', 'DAY TO HOUR', 'DAY TO MINUTE', 'DAY TO SECOND', 'HOUR TO MINUTE', 'HOUR TO SECOND', or 'MINUTE TO SECOND'. INTERVAL_PRECISION INTEGER ... where INTERVAL_PREVISION is not fractional-seconds precision (that's taken care of by another column that's already in the data type descriptor), but the precision of the leading field. If you said DAY(5) then INTERVAL_PRECISION = 5. Errors ------ The standard defines these errors: 22015 Interval field overflow 2200P Interval value out of range Use '22015' for everything except aggregate functions. Non-standard expressions ------------------------ The MySQL Reference Manual, 12.5. Date and Time Functions, describes non-standard keywords for DATE_ADD and DATE_SUB: MICROSECOND MICROSECONDS WEEK WEEKS QUARTER QUARTERS SECOND_MICROSECOND 'SECONDS.MICROSECONDS' MINUTE_MICROSECOND 'MINUTES.MICROSECONDS' MINUTE_SECOND 'MINUTES:SECONDS' HOUR_MICROSECOND 'HOURS.MICROSECONDS' HOUR_SECOND 'HOURS:MINUTES:SECONDS' HOUR_MINUTE 'HOURS:MINUTES' DAY_MICROSECOND 'DAYS.MICROSECONDS' DAY_SECOND 'DAYS HOURS:MINUTES:SECONDS' DAY_MINUTE 'DAYS HOURS:MINUTES' DAY_HOUR 'DAYS HOURS' YEAR_MONTH 'YEARS-MONTHS' Unfortunately these non-standard keywords have migrated to the pseudo-interval support. MySQL currently allows: mysql> select s1 + interval '1' year_month from td; +-------------------------------+ | s1 + interval '1' year_month | +-------------------------------+ | 2007-02-25 | +-------------------------------+ 1 row in set (0.00 sec) A similar extension exists for CREATE EVENT. Peter asked for it to be removed, without success. The recommendation now is: remove support for all the non-standard keywords. References ---------- SQL-99 Complete, Really pages 184ff. crash-me test results (all types are tested in crash-me) Feature requests: BUG#20844 interval datatype

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