There are two types of comments. One-line comments start with “–” two dashes and they end at the end of the line. First, we will start the server we previously created with “monetdbd start /home/fffovde/DBfarm1”. We will also start our mclient application with “mclient -u voc -d voc”. Then we can try such comment as:
sql>SELECT * more>FROM --some comment more>total limit 5;
We can also use multiple lines comment. They start with “/*”, and they end with “*/”.
sql>SELECT /* more>some comment more>here */ * FROM total LIMIT 5;
Identifiers and Keywords
Identifiers and Keywords are not case sensitive.
If we have an identifier that is the same word as keyword, then we should place quotes around it. We can have column named SELECT, if this identifier is inside of quotes. This also allows usage of spaces and special characters inside of our identifier.
SELECT 'value as "SELECT.;' ";
Identifiers can not start with % symbol.
SELECT 'value' as "%columnName";SELECT 'value' as "columknName";
String constants are delimited with single quotes like ‘string data’. If our text contains single quotes then such single quotes should be doubled, like ‘O’’Connor’.
SELECT 'O''Connor' AS columnName;
We can use UNICODE codes to create constants.
sql>SELECT U&'\0441\043F\0430\0441\0438\0431\043E' as "thank you";
With UESCAPE it is possible to change default escape sign:
sql>SELECT U&'*0441*043F*0430*0441*0438*0431*043E' UESCAPE '*' as "thank you";
These constants can be typed as strings, but will still be recognized as time constants.
Inside of strings we can use these special characters.
|\t – this will return TAB.|
\n – new line.
\r – carriage return.
\f – form feed.
\’ – single quote.
\\ – backslash.
If we want to disable such behavior, we can use raw strings. We just type “R” before string, and escape sequences will be ignored.
Data is expressed as scalar or a table. Scalars are constants, column references, operator results, function results, and subqueries that return one value. Column reference is written as “tableName.columnName”. We can omit “tableName” and only write “columnName” if there is no ambiguity.
Table name is written as “schemaName.tableName”. If there is no ambiguity, we can write only “tableName”.
We can also reference tables and columns through their aliases.