Grouping Data

GROUP BY clause Groups results by column name used with aggregate functions must come first when used with ORDER BY

Syntax SELECT count(au_id), state, city FROM authors GROUP BY state, city column in GROUP BY must be in SELECT list There can be multiple columns in GROUP BY. Where their list order is their order of precedence.

Limitations There must be only 1 value returned for each GROUP BY field. Should only use column names Due to these limitations, many vendors provide report generators

Grouping and Nulls Nulls are assigned a group of there own since they are, in a sense, their own datatype.

HAVING clause Same as WHERE clause except it allows aggregate functions Often used with GROUP BY Must come after GROUP BY when both are used in a query. Unlike WHERE, all the fields in the HAVING list must also be in the SELECT list.

Syntax for HAVING Select type, count(*) from titles GROUP BY type HAVING count(*) > 1 Results: type______ business4 mod_cook2

HAVING Multiple conditions can be specified through the use of AND, OR, or NOT keywords. Difference with WHERE functionality is that WHERE eliminates rows before grouping and HAVING does so after grouping.

Another syntax example SELECT pub_id, sum(advance), avg(price) FROM titles WHERE price >= 5 GROUP BY pub_id HAVING sun(advance) > 1500 and avg(price) < 20 ORDER BY pub_id

About Security As is the case in most shared environments, the DBMS also must implement a security mechanism that allows the setting of permissions to data and actions pertaining to that data. This is required to ensure data security.

GRANT Command This “grants” a user or group of users permission to manipulate specified data in specified ways. GRANT {ALL | privilege_list} ON {table_name | view_name [(col_list)]} TO {PUBLIC | user_list}

REVOKE Command This “revokes” a granted permission issued by GRANT from a specified user. REVOKE {ALL | privilege_list} ON {table_name | view_name [(col_list)]} FROM {PUBLIC | user_list} Tip: 1) You GRANT TO and REVOKE FROM. 2) The most recently issued statement supercedes all others.