cloud-fan commented on a change in pull request #33693:
URL: https://github.com/apache/spark/pull/33693#discussion_r686533489
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File path: docs/sql-ref-literals.md
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@@ -439,32 +439,93 @@ SELECT TIMESTAMP '1997-01' AS col;
An interval literal is used to specify a fixed period of time.
+#### ANSI style
+
+The ANSI SQL standard defines interval literals in the form:
+```sql
+INTERVAL [ <sign> ] <interval string> <interval qualifier>
+```
+where `<interval qualifier>` can be a single field or in the field-to-field
form:
+```sql
+<interval qualifier> ::= <start field> TO <end field> | <single field>
+```
+An interval literal can have either year-month or day-time interval type. The
interval sub-type defines format of `<interval string>`:
+```sql
+<interval string> ::= <quote> [ <sign> ] { <year-month literal> | <day-time
literal> } <quote>
+<year-month literal> ::= <years value> [ <minus sign> <months value> ] |
<months value>
+<day-time literal> ::= <day-time interval> | <time interval>
+<day-time interval> ::= <days value> [ <space> <hours value> [ <colon>
<minutes value> [ <colon> <seconds value> ] ] ]
+<time interval> ::= <hours value> [ <colon> <minutes value> [ <colon> <seconds
value> ] ]
+ | <minutes value> [ <colon> <seconds value> ]
+ | <seconds value>
+```
+
+Supported year-month interval literals and theirs formats:
Review comment:
can we just say
```
The field names are case insensitive and can be one of YEAR, MONTH, DAY,
HOUR, MINUTE, and SECOND.
```
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