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https://issues.apache.org/jira/browse/CALCITE-2980?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17657010#comment-17657010
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TJ Banghart commented on CALCITE-2980:
--------------------------------------
Leaving some notes on how we could address format strings that [~julianhyde]
and I thought through.
We’ll create a small utility {{class FormatModels}} for model parsing. This
should be testable on its own (without writing SQL) and distinct from the SQL
parser. Format elements will be “pluggable” sets supplied by the lexical
policy, conformance, or dialect class (TBD) which implement an {{interface
FormatModelElement}}. This interface should have methods to unparse a timestamp
and also to parse a string into a timestamp (returning the number of characters
consumed).
Using this utility we’ll convert format model string operands into a novel
{{FormatModel}} operator whose operands are the token identifiers that
represent the format string. For example, {{“%b-%Y”}} becomes
{{FormatModel(MONTH_ABV, MINUS, YEAR)}}. These tokens could then be written
using the format elements of the target dialect.
This should provide a generic solution for similar formatting/parsing functions
like {{FORMAT_TIMESTAMP}}, {{TO_DATE}}, {{PARSE_TIMESTAMP}}, etc...
> Implement the FORMAT clause of the CAST operator
> ------------------------------------------------
>
> Key: CALCITE-2980
> URL: https://issues.apache.org/jira/browse/CALCITE-2980
> Project: Calcite
> Issue Type: Bug
> Reporter: Julian Hyde
> Assignee: TJ Banghart
> 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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