inheritance structure
not null if expression is a CASE WHEN ... expression
not null if expression must be cast to another data type
not null if expression is a condition or an operation
not null if expression is a function or aggregate
a #GdaSqlParamSpec, or null if this is not a variable
not null if expression is a sub select statement (#GdaSqlStatementSelect or #GdaSqlStatementCompound)
a #GValue, or null. Please see specific note about this field.
Please see specific note about the @value field
This structure contains any expression, either as a value (the @value part is set), a variable (the @param_spec is set), or as other types of expressions.
Note 1 about the @value field: if the expression represents a string value in the SQL statement, the string itself must be represented as it would be in the actual SQL, ie. it should be escaped (accordingly to the escaping rules of the database which will use the SQL). For example a string representing the <userinput>'joe'</userinput> value should be <userinput>"'joe'"</userinput> and not <userinput>"joe"</userinput>.
Note 2 about the @value field: if the expression represents an SQL identifier (such as a table or field name), then the @value_is_ident should be set to true, and @value should be a string which may contain double quotes around SQL identifiers which also are reserved keywords or which are case sensitive.