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https://issues.apache.org/jira/browse/CALCITE-2980?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17043139#comment-17043139
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Julian Hyde commented on CALCITE-2980:
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I have a request for the implementation. When implementing {{CAST(<datetime> AS
<char string type> [FORMAT <template>])}}, please separate the parsing of the
format string from the format elements. Why? Because I think we will at some
point want to support other formatting functions, e.g. Oracle's {{TO_CHAR}},
and while the front end may be different, I would like these to use the same
back-end.
The back-end elements, e.g. 'Output the 4 digit representation of the year'
occur in most formatting functions.
If may be possible to use the format elements when converting strings to
datetime values (i.e. to implement functions like {{TO_DATE}} and {{TO_NUMBER}}
and {{CAST(<char string> AS <datetime type> [FORMAT <template>])}}).
Later, we could consider optimizing. Some options:
* If the format string is a literal, parse the format string into format
elements at prepare time.
* Generate Java code from the format elements and compile to Janino.
* If the format string is not a literal, is to cache the result of parsing in a
map from string to lists of format elements.
> Implement the FORMAT clause of the CAST operator (added in SQL:2016)
> --------------------------------------------------------------------
>
> Key: CALCITE-2980
> URL: https://issues.apache.org/jira/browse/CALCITE-2980
> Project: Calcite
> Issue Type: Bug
> Reporter: Julian Hyde
> Assignee: Rui Wang
> Priority: Major
>
> SQL:2016 adds an optional {{FORMAT format}} clause to the {{CAST}} operator.
> It is a standard way to do what functions like {{TO_DATE}}, {{TO_NUMBER}},
> {{TO_CHAR}}, {{TO_TIMESTAMP}} have done in an ad hoc way (and with differing
> specifications among databases).
> Here is an example:
> {code:java}
> cast('01-05-2017' as date format 'DD-MM-YYYY')
> {code}
> The following paragraphs are copied from IMPALA-4018, which describes
> implementing this in Impala. (That case also describes cases where the
> implementations of {{TO_TIMESTAMP}} etc. in Hive, Impala, Oracle and
> PostgreSQL are not consistent with each other. We should take note as we
> implement these functions in Calcite.)
> SQL:2016 defines the following datetime templates
> {noformat}
> <datetime template> ::=
> { <datetime template part> }...
> <datetime template part> ::=
> <datetime template field>
> | <datetime template delimiter>
> <datetime template field> ::=
> <datetime template year>
> | <datetime template rounded year>
> | <datetime template month>
> | <datetime template day of month>
> | <datetime template day of year>
> | <datetime template 12-hour>
> | <datetime template 24-hour>
> | <datetime template minute>
> | <datetime template second of minute>
> | <datetime template second of day>
> | <datetime template fraction>
> | <datetime template am/pm>
> | <datetime template time zone hour>
> | <datetime template time zone minute>
> <datetime template delimiter> ::=
> <minus sign>
> | <period>
> | <solidus>
> | <comma>
> | <apostrophe>
> | <semicolon>
> | <colon>
> | <space>
> <datetime template year> ::=
> YYYY | YYY | YY | Y
> <datetime template rounded year> ::=
> RRRR | RR
> <datetime template month> ::=
> MM
> <datetime template day of month> ::=
> DD
> <datetime template day of year> ::=
> DDD
> <datetime template 12-hour> ::=
> HH | HH12
> <datetime template 24-hour> ::=
> HH24
> <datetime template minute> ::=
> MI
> <datetime template second of minute> ::=
> SS
> <datetime template second of day> ::=
> SSSSS
> <datetime template fraction> ::=
> FF1 | FF2 | FF3 | FF4 | FF5 | FF6 | FF7 | FF8 | FF9
> <datetime template am/pm> ::=
> A.M. | P.M.
> <datetime template time zone hour> ::=
> TZH
> <datetime template time zone minute> ::=
> TZM
> {noformat}
> SQL:2016 also introduced the {{FORMAT}} clause for {{CAST}} which is the
> standard way to do string <> datetime conversions
> {noformat}
> <cast specification> ::=
> CAST <left paren>
> <cast operand> AS <cast target>
> [ FORMAT <cast template> ]
> <right paren>
> <cast operand> ::=
> <value expression>
> | <implicitly typed value specification>
> <cast target> ::=
> <domain name>
> | <data type>
> <cast template> ::=
> <character string literal>
> {noformat}
> For example:
> {noformat}
> CAST(<datetime> AS <char string type> [FORMAT <template>])
> CAST(<char string> AS <datetime type> [FORMAT <template>])
> cast(dt as string format 'DD-MM-YYYY')
> {noformat}
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