A value expression is one of the following:
A constant or literal value A column reference A positional parameter reference, in the body of a function definition or prepared statement A subscripted expression A field selection expression An operator invocation A function call An aggregate expression A window function call A type cast A collation expression A scalar subquery An array constructor A row constructor Another value expression in parentheses (used to group subexpressions and override precedence)Column References（列引用）
correlation.columnname users.username
Positional Parameters（位置参数， 同 BASH ）
For example, consider the definition of a function, dept, as:
CREATE FUNCTION dept(text) RETURNS dept
AS $$ SELECT * FROM dept WHERE name = $1 $$
LANGUAGE SQL;
$1 表示参数里的第一个参数。
Subscripts， 有点像python 列表切片
expression[subscript]
Operator Invocations
Function Calls
function_name ([expression [, expression ... ]] ) sqrt(2)
似懂非懂，不继续流了 J J J
转载于:https://www.cnblogs.com/songlihong/p/5368511.html
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