The Postgresql formatting 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 9-20 lists 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.

A single-argument to_timestamp function is also available; it accepts a double precision argument and converts from Unix epoch (seconds since 1970-01-01 00:00:00+00) to timestamp with time zone. (Integer Unix epochs are implicitly cast to double precision.)

In a to_char output 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 9-21 shows the template patterns available for formatting date and time values.

Modifiers can be applied to any template pattern to alter its behavior. For example,FMMonth is the Month pattern with the FM modifier. Table 9-22 shows the modifier patterns for date/time formatting.

Usage notes for date/time formatting:

• FM suppresses leading zeroes and trailing blanks that would otherwise be added to make the output of a pattern be fixed-width. In Postgresql,FM modifies only the next specification,while in Oracle FM affects all subsequent specifications,and repeated FM modifiers toggle fill mode on and off.

• TM does not include trailing blanks.

• to_timestamp and to_date skip multiple blank spaces in the input string unless the FX option is used. For example,to_timestamp('2000JUN','YYYY MON') works,but to_timestamp('2000JUN','FXYYYY MON') returns an error because to_timestamp expects one space only. FX must be specified as the first item in the template.

• Ordinary text is allowed in to_char templates 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',the YYYY will be replaced by the year data,but the single Y in Year will not be. In to_date,to_number,and to_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 E'//"YYYY Month//"'. (Two backslashes are necessary because the backslash has special meaning when using the escape string Syntax.)

• The YYYY conversion from string to timestamp or date has a restriction when processing years with more than 4 digits. You must use some non-digit character or template after YYYY,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,like to_date('20000-1131','YYYY-MMDD') or to_date('20000Nov31','YYYYMonDD').

• In conversions from string to timestamp or date,the CC (century) field is ignored if there is a YYY,YYYY or Y,YYY field. If CC is used with YY or Y then the year is computed as (CC-1)*100+YY.

• An ISO week date (as distinct from a Gregorian date) can be specified to to_timestamp and to_date in one of two ways:

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

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

  Attempting to construct a date using a mixture of ISO week and Gregorian date fields is nonsensical,and will cause an error. In the context of an ISO 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. Users should avoid mixing Gregorian and ISO date specifications.

• In a conversion from string to timestamp,millisecond (MS) or microsecond (US) values are used as the seconds digits after the decimal point. For example to_timestamp('12:3','SS:MS') is not 3 milliseconds,but 300,because the conversion counts it as 12 + 0.3 seconds. This means for the format SS:MS,the input values 12:3,12:30,and 12:300 specify the same number of milliseconds. To get three milliseconds,one must use 12:003,which the conversion counts as 12 + 0.003 = 12.003 seconds.

  Here is a more complex example: to_timestamp('15:12:02.020.001230','HH: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 the extract(isodow from ...) function,but to_char(...,'D')'s does not match extract(dow from ...)'s day numbering.

• to_char(interval) formats HH and HH12 as shown on a 12-hour clock,i.e. zero hours and 36 hours output as 12,while HH24 outputs the full hour value,which can exceed 23 for intervals.

Table 9-23 shows the template patterns available for formatting numeric values.

Usage notes for numeric formatting:

• A sign formatted using SG,PL,or MI is not anchored to the number; for example,to_char(-12,'MI9999') produces '-12' but to_char(-12,'S9999') produces '-12'. The Oracle implementation does not allow the use of MI before 9,but rather requires that 9 precede MI.

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

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

• PL,SG,and TH are Postgresql extensions.

• V effectively multiplies the input values by 10^n,where n is the number of digits following V. to_char does not support the use of V combined with a decimal point (e.g.,99.9V99 is 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.,9.99EEEE is a valid pattern).

Certain modifiers can be applied to any template pattern to alter its behavior. For example,FM9999 is the 9999 pattern with the FM modifier. Table 9-24 shows the modifier patterns for numeric formatting.

Table 9-25 shows some examples of the use of the to_char function.