Postgresql流水帐（第二天）： Value Expressions

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