9.8. 型別轉換函式

ThePostgreSQLformatting functions provide a powerful set of tools for converting various data types (date/time, integer, floating point, numeric) to formatted strings and for converting from formatted strings to specific data types.Table 9.23lists them. These functions all follow a common calling convention: the first argument is the value to be formatted and the second argument is a template that defines the output or input format.

Table 9.23. Formatting Functions

Note

There is also a single-argumentto_timestampfunction; seeTable 9.30.

Tip

to_timestampandto_dateexist to handle input formats that cannot be converted by simple casting. For most standard date/time formats, simply casting the source string to the required data type works, and is much easier. Similarly,to_numberis unnecessary for standard numeric representations.

In ato_charoutput template string, there are certain patterns that are recognized and replaced with appropriately-formatted data based on the given value. Any text that is not a template pattern is simply copied verbatim. Similarly, in an input template string (for the other functions), template patterns identify the values to be supplied by the input data string.

Table 9.24shows the template patterns available for formatting date and time values.

Table 9.24. Template Patterns for Date/Time Formatting

Modifiers can be applied to any template pattern to alter its behavior. For example,FMMonthis theMonthpattern with theFMmodifier.Table 9.25shows the modifier patterns for date/time formatting.

Table 9.25. Template Pattern Modifiers for Date/Time Formatting

Usage notes for date/time formatting:

• FMsuppresses leading zeroes and trailing blanks that would otherwise be added to make the output of a pattern be fixed-width. InPostgreSQL,FMmodifies only the next specification, while in OracleFMaffects all subsequent specifications, and repeatedFMmodifiers toggle fill mode on and off.

• TMdoes not include trailing blanks.to_timestampandto_dateignore theTMmodifier.

• to_timestampandto_dateskip multiple blank spaces in the input string unless theFXoption is used. For example,to_timestamp('2000 JUN', 'YYYY MON')works, butto_timestamp('2000 JUN', 'FXYYYY MON')returns an error becauseto_timestampexpects one space only.FXmust be specified as the first item in the template.

• Ordinary text is allowed into_chartemplates and will be output literally. You can put a substring in double quotes to force it to be interpreted as literal text even if it contains pattern key words. For example, in'"Hello Year "YYYY', theYYYYwill be replaced by the year data, but the singleYinYearwill not be. Into_date,to_number, andto_timestamp, double-quoted strings skip the number of input characters contained in the string, e.g."XX"skips two input characters.

• If you want to have a double quote in the output you must precede it with a backslash, for example'\"YYYY Month\"'.

• Into_timestampandto_date, if the year format specification is less than four digits, e.g.YYY, and the supplied year is less than four digits, the year will be adjusted to be nearest to the year 2020, e.g.95becomes 1995.

• Into_timestampandto_date, theYYYYconversion has a restriction when processing years with more than 4 digits. You must use some non-digit character or template afterYYYY, otherwise the year is always interpreted as 4 digits. For example (with the year 20000):to_date('200001131', 'YYYYMMDD')will be interpreted as a 4-digit year; instead use a non-digit separator after the year, liketo_date('20000-1131', 'YYYY-MMDD')orto_date('20000Nov31', 'YYYYMonDD').

• Into_timestampandto_date, theCC(century) field is accepted but ignored if there is aYYY,YYYYorY,YYYfield. IfCCis used withYYorYthen the result is computed as that year in the specified century. If the century is specified but the year is not, the first year of the century is assumed.

• Into_timestampandto_date, weekday names or numbers (DAY,D, and related field types) are accepted but are ignored for purposes of computing the result. The same is true for quarter (Q) fields.

• Into_timestampandto_date, an ISO 8601 week-numbering date (as distinct from a Gregorian date) can be specified in one of two ways:

  • Year, week number, and weekday: for exampleto_date('2006-42-4', 'IYYY-IW-ID')returns the date2006-10-19. If you omit the weekday it is assumed to be 1 (Monday).

  • Year and day of year: for exampleto_date('2006-291', 'IYYY-IDDD')also returns2006-10-19.

  Attempting to enter a date using a mixture of ISO 8601 week-numbering fields and Gregorian date fields is nonsensical, and will cause an error. In the context of an ISO 8601 week-numbering year, the concept of a“month”or“day of month”has no meaning. In the context of a Gregorian year, the ISO week has no meaning.

  Caution

  Whileto_datewill reject a mixture of Gregorian and ISO week-numbering date fields,to_charwill not, since output format specifications likeYYYY-MM-DD (IYYY-IDDD)can be useful. But avoid writing something likeIYYY-MM-DD; that would yield surprising results near the start of the year. (SeeSection 9.9.1for more information.)

• Into_timestamp, millisecond (MS) or microsecond (US) fields are used as the seconds digits after the decimal point. For exampleto_timestamp('12.3', 'SS.MS')is not 3 milliseconds, but 300, because the conversion treats it as 12 + 0.3 seconds. So, for the formatSS.MS, the input values12.3,12.30, and12.300specify the same number of milliseconds. To get three milliseconds, one must write12.003, which the conversion treats as 12 + 0.003 = 12.003 seconds.

  Here is a more complex example:to_timestamp('15:12:02.020.001230', 'HH24:MI:SS.MS.US')is 15 hours, 12 minutes, and 2 seconds + 20 milliseconds + 1230 microseconds = 2.021230 seconds.

• to_char(..., 'ID')'s day of the week numbering matches theextract(isodow from ...)function, butto_char(..., 'D')'s does not matchextract(dow from ...)'s day numbering.

• to_char(interval)formatsHHandHH12as shown on a 12-hour clock, for example zero hours and 36 hours both output as12, whileHH24outputs the full hour value, which can exceed 23 in anintervalvalue.

Table 9.26shows the template patterns available for formatting numeric values.

Table 9.26. Template Patterns for Numeric Formatting

Usage notes for numeric formatting:

• A sign formatted usingSG,PL, orMIis not anchored to the number; for example,to_char(-12, 'MI9999')produces'- 12'butto_char(-12, 'S9999')produces' -12'. The Oracle implementation does not allow the use ofMIbefore9, but rather requires that9precedeMI.

• 9results in a value with the same number of digits as there are9s. If a digit is not available it outputs a space.

• THdoes not convert values less than zero and does not convert fractional numbers.

• PL,SG, andTHarePostgreSQLextensions.

• Vwithto_charmultiplies the input values by10^n, where_n_is the number of digits followingV.Vwithto_numberdivides in a similar manner.to_charandto_numberdo not support the use ofVcombined with a decimal point (e.g.,99.9V99is not allowed).

• EEEE(scientific notation) cannot be used in combination with any of the other formatting patterns or modifiers other than digit and decimal point patterns, and must be at the end of the format string (e.g.,9.99EEEEis a valid pattern).

Certain modifiers can be applied to any template pattern to alter its behavior. For example,FM9999is the9999pattern with theFMmodifier.Table 9.27shows the modifier patterns for numeric formatting.

Table 9.27. Template Pattern Modifiers for Numeric Formatting

Table 9.28shows some examples of the use of theto_charfunction.

Table 9.28. to_charExamples