特殊语法形式

为了兼容SQL标准和PostgreSQL，以下函数可以被支持，同时等价于常规的内置函数：

特殊形式 等价于
CURRENT_CATALOG current_catalog()
CURRENT_DATE current_date()
CURRENT_ROLE current_user()
CURRENT_SCHEMA current_schema()
CURRENT_TIMESTAMP current_timestamp()
CURRENT_TIME current_time()
CURRENT_USER current_user()
EXTRACT( FROM ) extract(“”, )
EXTRACT_DURATION(FROM ) extract_duration(“”,)
OVERLAY( PLACING FROM FOR ) overlay(,, , )
OVERLAY( PLACING FROM ) overlay(,, )
POSITION( IN ) strpos(, )
SESSION_USER current_user()
SUBSTRING(FORFROM ) substring(,, )
SUBSTRING( FOR ) substring(, 1, )
SUBSTRING(FROMFOR) substring(,, )
SUBSTRING( FROM ) substring(, )
TRIM( FROM ) btrim(, )
TRIM(, ) btrim(, )
TRIM(FROM ) btrim()
TRIM(LEADING FROM ) ltrim(, )
TRIM(LEADING FROM ) ltrim()
TRIM(TRAILING FROM ) rtrim(, )
TRIM(TRAILING FROM ) rtrim()
USER current_user()