9.8. 型別轉換函式
PostgreSQL 格式化函數提供了一套功能強大的工具,用於將各種資料型別(日期/時間、整數、浮點數、數字)轉換為格式化的字串,以及從格式化字串轉換為特定資料型別。Table 9.24 列出了這些函數,而這些函數都遵循一個通用的呼叫約定:第一個參數是要格式化的值,第二個參數是定義輸出或輸入格式的樣板。
Table 9.24. Formatting Functions
提醒 還有一個單一參數 to_timestamp 函數; 請參閱 Table 9.31。
小技巧 存在有 to_timestamp 和 to_date 來處理無法透過簡單轉換進行轉換的輸入格式。對於大多數標準日期/時間格式,只需將來源字串強制轉換為所需的資料型別即可,並且非常容易。同樣地,對於標準數字表示形式,to_number 也不是必要的。
在 to_char 輸出樣版字串中,基於給予值識別並替換為某些格式資料的某些樣式。 非樣板的任何文字都將被逐字複製。同樣地,在輸入樣板字串(用於其他功能)中,樣板標識輸入資料字串要提供的值。如果樣板字串中存在不是樣板的字串,則只需跳過輸入資料字串中的相對應字元(無論它們是否等於樣板字串字元)。
Table 9.25 shows the template patterns available for formatting date and time values.
Table 9.25. Template Patterns for Date/Time Formatting
Modifiers can be applied to any template pattern to alter its behavior. For example, FMMonth
is the Month
pattern with the FM
modifier. Table 9.26 shows the modifier patterns for date/time formatting.
Table 9.26. Template Pattern Modifiers 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 OracleFM
affects all subsequent specifications, and repeatedFM
modifiers toggle fill mode on and off.TM
does not include trailing blanks.to_timestamp
andto_date
ignore theTM
modifier.to_timestamp
andto_date
skip multiple blank spaces at the beginning of the input string and around date and time values unless theFX
option is used. For example,to_timestamp(' 2000 JUN', 'YYYY MON')
andto_timestamp('2000 - JUN', 'YYYY-MON')
work, butto_timestamp('2000 JUN', 'FXYYYY MON')
returns an error becauseto_timestamp
expects only a single space.FX
must be specified as the first item in the template.A separator (a space or non-letter/non-digit character) in the template string of
to_timestamp
andto_date
matches any single separator in the input string or is skipped, unless theFX
option is used. For example,to_timestamp('2000JUN', 'YYYY///MON')
andto_timestamp('2000/JUN', 'YYYY MON')
work, butto_timestamp('2000//JUN', 'YYYY/MON')
returns an error because the number of separators in the input string exceeds the number of separators in the template.If
FX
is specified, a separator in the template string matches exactly one character in the input string. But note that the input string character is not required to be the same as the separator from the template string. For example,to_timestamp('2000/JUN', 'FXYYYY MON')
works, butto_timestamp('2000/JUN', 'FXYYYY MON')
returns an error because the second space in the template string consumes the letterJ
from the input string.A
TZH
template pattern can match a signed number. Without theFX
option, minus signs may be ambiguous, and could be interpreted as a separator. This ambiguity is resolved as follows: If the number of separators beforeTZH
in the template string is less than the number of separators before the minus sign in the input string, the minus sign is interpreted as part ofTZH
. Otherwise, the minus sign is considered to be a separator between values. For example,to_timestamp('2000 -10', 'YYYY TZH')
matches-10
toTZH
, butto_timestamp('2000 -10', 'YYYY TZH')
matches10
toTZH
.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 template patterns. For example, in'"Hello Year "YYYY'
, theYYYY
will be replaced by the year data, but the singleY
inYear
will not be. Into_date
,to_number
, andto_timestamp
, literal text and double-quoted strings result in skipping the number of characters contained in the string; for example"XX"
skips two input characters (whether or not they areXX
).Tip
Prior to PostgreSQL 12, it was possible to skip arbitrary text in the input string using non-letter or non-digit characters. For example,
to_timestamp('2000y6m1d', 'yyyy-MM-DD')
used to work. Now you can only use letter characters for this purpose. For example,to_timestamp('2000y6m1d', 'yyyytMMtDDt')
andto_timestamp('2000y6m1d', 'yyyy"y"MM"m"DD"d"')
skipy
,m
, andd
.If you want to have a double quote in the output you must precede it with a backslash, for example
'\"YYYY Month\"'
. Backslashes are not otherwise special outside of double-quoted strings. Within a double-quoted string, a backslash causes the next character to be taken literally, whatever it is (but this has no special effect unless the next character is a double quote or another backslash).In
to_timestamp
andto_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.95
becomes 1995.In
to_timestamp
andto_date
, theYYYY
conversion 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')
.In
to_timestamp
andto_date
, theCC
(century) field is accepted but ignored if there is aYYY
,YYYY
orY,YYY
field. IfCC
is used withYY
orY
then 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.In
to_timestamp
andto_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.In
to_timestamp
andto_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 example
to_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 example
to_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
While
to_date
will reject a mixture of Gregorian and ISO week-numbering date fields,to_char
will 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. (See Section 9.9.1 for more information.)In
to_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
, and12.300
specify 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)
formatsHH
andHH12
as shown on a 12-hour clock, for example zero hours and 36 hours both output as12
, whileHH24
outputs the full hour value, which can exceed 23 in aninterval
value.
Table 9.27 shows the template patterns available for formatting numeric values.
Table 9.27. Template Patterns for Numeric Formatting
Usage notes for numeric formatting:
0
specifies a digit position that will always be printed, even if it contains a leading/trailing zero.9
also specifies a digit position, but if it is a leading zero then it will be replaced by a space, while if it is a trailing zero and fill mode is specified then it will be deleted. (Forto_number()
, these two pattern characters are equivalent.)The pattern characters
S
,L
,D
, andG
represent the sign, currency symbol, decimal point, and thousands separator characters defined by the current locale (see lc_monetary and lc_numeric). The pattern characters period and comma represent those exact characters, with the meanings of decimal point and thousands separator, regardless of locale.If no explicit provision is made for a sign in
to_char()
's pattern, one column will be reserved for the sign, and it will be anchored to (appear just left of) the number. IfS
appears just left of some9
's, it will likewise be anchored to the number.A sign formatted using
SG
,PL
, orMI
is 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 ofMI
before9
, but rather requires that9
precedeMI
.)TH
does not convert values less than zero and does not convert fractional numbers.PL
,SG
, andTH
are PostgreSQL extensions.In
to_number
, if non-data template patterns such asL
orTH
are used, the corresponding number of input characters are skipped, whether or not they match the template pattern, unless they are data characters (that is, digits, sign, decimal point, or comma). For example,TH
would skip two non-data characters.V
withto_char
multiplies the input values by10^
n
, wheren
is the number of digits followingV
.V
withto_number
divides in a similar manner.to_char
andto_number
do not support the use ofV
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, FM99.99
is the 99.99
pattern with the FM
modifier. Table 9.28 shows the modifier patterns for numeric formatting.
Table 9.28. Template Pattern Modifiers for Numeric Formatting
Table 9.29 shows some examples of the use of the to_char
function.
Table 9.29. to_char
Examples
to_char
ExamplesLast updated