Explanation : 1- DML stands for data manipulation language. The SQL commands that deals with the manipulation of data present in the database belong to DML or Data Manipulation Language including retrieving, storing, modification, deletion, insertion and updation of data in a database. Examples of DML: SELECT, INSERT, UPDATE, DELETE.
2- DDL - Data Definition Language - This commands in SQL includes the creation, deletion and modification of structure of database objects. Examples of DDL commands: CREATE, DROP, ALTER, TRUNCATE, COMMENT, RENAME. It provides the facility of accessing data from a database.
3- Functions of a DBA include: Schema deﬁnition, Storage structure and access-method deﬁnition, Schema and physical-organization modiﬁcation, Granting of authorization for data access, Routine maintenance.
4- Schema: The term "schema" refers to the organization of data as a blueprint of how the database is constructed (divided into database tables in the case of relational databases). The formal definition of a database schema is a set of formulas (sentences) called integrity constraints imposed on a database.
So, option (A) is correct.
Explanation : In relational database model:
(a) NULL values can be used to opt a tuple out of enforcement of a foreign key. Correct
(b) Suppose that table T has only one candidate key. If Q is in 3NF, then it is also in BCNF. Correct
(c) The difference between the project operator (Π) in relational algebra and the SELECT keyword in SQL is that if the resulting table/set has more than one occurrences of the same tuple, then Π will return only one of them, while SQL SELECT will return all. Correct
So, option (D) is correct.
Explanation : SELECT A FROM R; legal query SELECT A, COUNT(*) FROM R; Illegal query because we can't SELECT A, COUNT(*) unless it is grouped by A. SELECT A, COUNT(*) FROM R GROUP BY A; legal query SELECT A, B, COUNT(*) FROM R GROUP BY A, B; legal query So, option (B) is correct.