Structured Query Language (SQL) provides the ability to create and define relational database objects. After these objects are defined, the language permits one to add data to these objects. Once data has been added, one can modify, retrieve, or delete that data. The language provides the capability of defining what type of authority one might have when accessing the data.
Data Definition Language
As the name implies, there is a group of SQL statements that allows one to define the relational structures that will manage the data placed in them. The “CREATE” statements brings Relational Database Management System (RDMS) objects into existence. The types of objects one can create are STOGROUP, Database, Table space, Table, Index, View, Synonym, and Alias. The definitions of these objects are as follows:
STOGROUP: A storage group is a list of disk volume names to which one can assign a name. One defines the list of disk volumes and assigns the STOGROUP name with the Create STOGROUP statement.
Database: A database is a logical structure in which tables and indexes are later created. The database is defined and associated with a STOGROUP with a Create Database statement.
Tablespace: A tablespace is an area on disk that is allocated and formatted by the Create Table space statement.
Table: A table is an organizational structure which is defined in a Create Table statement. In this statement, the data attributes are defined by column, giving each column its own unique name within the table.
Index: A index is used in conjuction with the “Primary Key” parameter of the Create Table statement. It is made with the Create Index statement and provides the duplicate record-checking necessary for a unique key.
View: A view is an alternative perspective of the data present in a database. It is made with the Create View statement and can represent a subset of the columns defined in a table. It can also represents a set of columns combined from more than one table.
Synonym: The Create Synonym statement defines an unqualified name for a table or a view.
Alias: The Create Alias statement defines an alternate qualified name for a table or a view.
After a table is created, additional columns may be added with an Alter Table statement. Any RDMS object that was made with a create statement can be removed with a drop statement.
In order to define RDMS objects, one needs various levels of authority. The following is a list of authority levels that can be granted to a user ID to operate on a designated database.
DBADM Database administrator authority
DBCTRL Database control authority
DBMAINT Database maintenance authority
CREATETS Create Table space Authority
CREATETAB Create Table authority
DROP Drop authority on a database or subordinate objects
Data Manipulation Language
There are four SQL data manipulation statements(DML) available: Insert, Select, Update, and Delete. After tables are defined, they are ready to store data. Data is added to tables through the SQL Insert statement. Once data has been inserted into a table, it can be retrieved by the use of the Select statement. Data stored in a table can be modified by executing the SQL Update statement. Data can be deleted from a table by using the SQL Delete statement.
The SQL statements perform RDMS operations that can affect only one row at a time if desired. The same statements can, if required, affect many or all of the rows in a table. It is possible to select one row and insert it into another with one statement. It is also just as easy to select all of the rows from one table and insert all of them into another with a single statement. The same scope of operation applied to the update and delete statements. The scope of operation is controlled by the use of the WHERE clause. The operation will affect only the rows that satisfy the search condition. When no search condition specified, the entire table is affected.
There are additional language elements available that provide the ability to process the table data while it is being retrieved. In addition, there are a variety of functions that modify the value of the data that is returned in a query. There are column functions that act on all of the values of the selected rows for a specified column and return a single answer. There are also scalar functions that return a specific answer for each row that satisfies the search condition.
As mentioned previously, SQL provides the ability to filter what data is retrieved in a select statement by including the WHERE clause. The WHERE clause specifies a variety of comparisons between two values. The values could be column values or the result of an operation involving more than one column or a constant. The comparison operation are the same as those used in COBOL, with the exception of two additional operators. The first is the IN operator that compares a single value has a match in the specified list of values. The other is the LIKE operator, in which you can specify a value string that includes “wildcard” characters in such a manner that you can select rows of a table where column values are similar to the extent you require.
SQL provides four arithmetic operations : addition, subtraction, multiplication, and division. An arithmetic expression may involve any combination of column name or numbers. The arithmetic expression may itself be used as a column name or in a Select, Insert, Update, or Delete statement.
SQL provides the ability to sort the data retrieved from a table via the ORDER BY clause. In this clause, you can specify one or more sort column names as well as if each sort key is ascending or descending.
SQL also provides the ability to perform set manipulation operations. Using SQL, one can SELECT the intersection of two or more sets of data by coding a JOIN. A JOIN is any SELECT statement that has more than one DBMS object listed in its FROM clause. One can combine different sets of data by using the UNION operator. Other set manipulations can be executed by combining different operators and search conditions.
The Following are the most frequently asked questions....
Q1) What RDMS objects are created with the SQL CREATE statements?
A1) The SQL CREATE statements are used to create the following objects:
STOGROUP A storage group
DATABASE A logical collection of tables
TABLESPACE An area that stores tables
TABLE A data structure organized by a specified columns
INDEX An alternate path to a table data
VIEW An alternate representation of one or more tables
SYNONYM An alternate name for local table or view
ALIAS An alternate name for a table definition which may be local or remote, existence or nonexistent
Q2) What RDMS objects are required before you can create a table?
A2) Before you can create a table, you need an existing database and tablespace.
Q3) In what RDMS object does one first list column names?
A3) One first uses the column name in the CREATE TABLE statement.
Q4) What is the syntax for a CREATE TABLE statement?
A4) CREATE TABLE table name
(column name list
primary key (column name))
in database-name, tablespace-name.
Q5) Can one add columns to a table after it has been defined?
A5) Yes, one can add column to a table after it has been defined by using the SQL ALTER TABLE statement.
Q6) Where in a table are added columns located?
A6) The new columns are added to the end of the table.
Q7) After a table is defined, can columns be removed?
A7) The only way to remove columns from an existing table involves a migration program that extracts only the desired
columns of data, redefining the table without the unwanted columns, then populating the new table. One have to handle
all the old table’s dependents programmatically.
Q8) Which RDMS objects can you change with the SQL ALTER statements?
A8) The SQL ALTER statement can change a table index, a table, a tablespace, or a STOGROUP.
Q9) What authority is required to create a table?
A9) In order to create tables, one needs CREATETAB privileges.
Q10) What is minimum authority required for one to create a tablespace?
A10) In order to create tablespaces, one needs CREATETS privileges.
Q11) When is it necessary to create a table index?
A11) It is necessary to create a table index whenever you want to enforce the uniqueness of the table’s primary key.
Q12) What is a synonym?
A12) A synonym is an unqualified alternative name for a table or view.
Q13) What is a foreign key?
A13) A foreign key is the key defined in one table to reference the primary key of a reference table. This foreign key must
have the same structure as the reference table’s primary key.
Q14) What is referential integrity?
A14) Referential integrity is the automatic enforcement of referential constraints that exist between a reference table and a
referencing table. When referential integrity is enforced, the value of a foreign key exists as a primary key value in the
reference table. In other words, when referential integrity is enforced, all of the foreign key values in, for example, the
“department code” column in an “employee” table exist as primary key values in a “department” table.
Q15) What are the column name qualifiers?
A15) A column name qualifier are used as a table designator to avoid ambiguity when the column names referenced exists
in more than one table used in the SQL statement. Column name qualifiers are also used in correlated references.
Q16) What is a correlation name?
A16) A correlation name is a special type of column designator that connects specific columns in the various levels of a
multilevel SQL query.
Q17) What is a results table?
A17) A result table is the product of a query against one or more tables or views (i.e., it is the place that holds the results of a
query).
Q18) What is a cursor?
A18) A cursor is a named control structure used to make a set of rows available to a program. DB2 is the relational database
system that runs in an MVS environment. It was developed by IBM and interfaces with SQL. With the use of SQL
DB2, databases can be accessed by a wide range of host languages. SQL is the relational database " application
language " that interfaces with DB2. Because of its capabilities, SQL and, in turn, DB2 have gained considerable
acceptance. Thus, a working knowledge of DB2 increases one's marketability.
Q19) What is the basic difference between a join and a union?
A19) A join selects columns from 2 or more tables. A union selects rows.
Q20) What is normalization and what are the five normal forms?
A20) Normalization is a design procedure for representing data in tabular format. The five normal forms are progressive
rules to represent the data with minimal redundancy.
Q21) What are foreign keys?
A21) These are attributes of one table that have matching values in a primary key in another table, allowing for relationships
between tables.
Q22) Describe the elements of the SELECT query syntax?
A22) SELECT element FROM table WHERE conditional statement.
Q23) Explain the use of the WHERE clause?
A23) WHERE is used with a relational statement to isolate the object element or row.
Q24) What techniques are used to retrieve data from more than one table in a single SQL statement?
A24) Joins, unions and nested selects are used to retrieve data.
Q25) What is a view? Why use it?
A25) A view is a virtual table made up of data from base tables and other views, but not stored separately.
Q26) Explain an outer join?
A26) An outer join includes rows from tables when there are no matching values in the tables.
Q27) What is a subselect? Is it different from a nested select?
A27) A subselect is a select which works in conjunction with another select. A nested select is a kind of subselect where the
inner select passes to the where criteria for the outer select.
Q28) What is the difference between group by and order by?
A28) Group by controls the presentation of the rows, order by controls the presentation of the columns for the results of the
SELECT statement.
Q29) What keyword does an SQL SELECT statement use for a string search?
A29) The LIKE keyword allows for string searches. The % sign is used as a wildcard.
Q30) What are some SQL aggregates and other built-in functions?
A30) The common aggregate, built-in functions are AVG, SUM, MIN, MAX, COUNT and DISTINCT.
Q31) How is the SUBSTR keyword used in SQL?
A31) SUBSTR is used for string manipulation with column name, first position and string length used as arguments. E.g.
SUBSTR (NAME, 1 3) refers to the first three characters in the column NAME.
Q32) Explain the EXPLAIN statement?
A32) The explain statement provides information about the optimizer's choice of access path of the SQL.
Q33) What is referential integrity?
A33) Referential integrity refers to the consistency that must be maintained between primary and foreign keys, i.e. every
foreign key value must have a corresponding primary key value.
Q34) What is a NULL value? What are the pros and cons of using NULLS?
A34) A NULL value takes up one byte of storage and indicates that a value is not present as opposed to a space or zero
value. It's the DB2 equivalent of TBD on an organizational chart and often correctly portrays a business situation.
Unfortunately, it requires extra coding for an application program to handle this situation.
Q35) What is a synonym? How is it used?
A35) A synonym is used to reference a table or view by another name. The other name can then be written in the
application code pointing to test tables in the development stage and to production entities when the code is migrated.
The synonym is linked to the AUTHID that created it.
Q36) What is an alias and how does it differ from a synonym?
A36) An alias is an alternative to a synonym, designed for a distributed environment to avoid having to use the location
qualifier of a table or view. The alias is not dropped when the table is dropped.
Q37) When can an insert of a new primary key value threaten referential integrity?
A37) Never. New primary key values are not a problem. However, the values of foreign key inserts must have
corresponding primary key values in their related tables. And updates of primary key values may require changes in
foreign key values to maintain referential integrity.
Q38) What is the difference between static and dynamic SQL?
A38) Static SQL is hard-coded in a program when the programmer knows the statements to be executed. For dynamic SQL
the program must dynamically allocate memory to receive the query results.
Q39) Compare a subselect to a join?
A39) Any subselect can be rewritten as a join, but not vice versa. Joins are usually more efficient as join rows can be
returned immediately, subselects require a temporary work area for inner selects results while processing the outer
select.
Q40) What is the difference between IN subselects and EXISTS subselect?
A40) If there is an index on the attributes tested an IN is more efficient since DB2 uses the index for the IN. (IN for index is
the mnemonic).
Q41) What is a Cartesian product?
A41) A Cartesian product results from a faulty query. It is a row in the results for every combination in the join tables.
Q42) What is a tuple?
A42) A tuple is an instance of data within a relational database.
Q43) What is the difference between static and dynamic SQL?
A43) Static SQL is compiled and optimized prior to its execution; dynamic is compiled and optimized during execution.
Q44) Any SQL implementation covers data types in couple of main categories. Which of the following are those data
types ? (Check all that apply)
A). NUMERIC
B). CHARACTER
C). DATE AND TIME
D). BLOBS E. BIT
A44) A,B,C. Not all SQL implementations have a BLOB or a BIT data types.
Q45) We have a table with a CHARACTER data type field. We apply a ">" row comparison between this field and
another CHARACTER field in another table. What will be the results for records with field value of NULL?
(Check one that applies the best)
A. TRUE
B. B. FALSE
C. C. UNKNOWN
D. D. Error.
E. E. Those records will be ignored
A45) C. NULL in a row when compared will give an UNKNOWN result.
Q46) Any database needs to go through a normalization process to make sure that data is represented only once. This
will eliminate problems with creating or destroying data in the database. The normalization process is done
usually in three steps which results in first, second and third normal forms. Which best describes the process to
obtain the third normal form? (Check one that applies the best)
A. Each table should have related columns.
B. Each separate table should have a primary key.
C. We have a table with multi-valued key. All columns that are dependent on only one or on some of the keys should be moved in a different table.
D. If a table has columns not dependent on the primary keys, they need to be moved in a separate table.
E. E. Primary key is always UNIQUE and NOT NULL.
A46) D. All columns in a table should be dependent on the primary key. This will eliminate transitive dependencies in
which A depends on B, and B depends on C, but we're not sure how C depends on A.
Q47) SQL can be embedded in a host program that uses a relational database as a persistent data repository. Some of
the most important pre-defined structures for this mechanism are SQLDA ("SQL Descriptor Area") and
SQLCA ("SQL Communications Area") SQLCA contains two structures - SQLCODE and SQLSTATE.
SQLSTATE is a standard set of error messages and warnings in which the first two characters defines the class
and the last three defines the subclass of the error. Which of the following SQLSTATE codes is interpreted as
"No data returned"?(Check one that applies the best)
A). 00xxx
B). 01xxx
C). 02xxx
D). 22xxx
E). 2Axxx
A47) C. 00 - is successful completion, 01 - warnings, 22 - is data exception and 2A is syntax error. The SQLSTATE code
format returned for "No data returned" is "02xxx".
Q48) What are common SQL abend codes? (e.g. : 0,100 etc.,)
A48) -818 time stamp mismatch
-180 wrong data moved into date field
Q49) What is meant by dynamic SQL?
A49) Dynamic SQL are SQL statements that are prepared and executed within a program while the program is executing.
The SQL source is contained in host variables rather than being hard coded into the program. The SQL statement may
change from execution to execution.
Q50) What is meant by embedded SQL?
A50) They are SQL statements that are embedded with in application program and are prepared during the program
preparation process before the program is executed. After it is prepared, the statement itself does not change(although
values of host variables specified within the statement might change).
Q51) What is meant by entity integrity?
A51) Entity integrity is when the primary key is in fact unique and not null.
Q52) What will EXPLAIN do?
A52) EXPLAIN obtains information (which indexes are used, whether sorting is necessary, which level of locking is
applied) about how SQL statements in the DBRM will be executed, inserting this information into the
“X”.PLAN.TABLE where the “X” is the authorization ID of the owner of the plan.
Q53) What is the foreign key?
A53) A foreign key is a column (or combination of columns) in a table whose values are required to match those of the
primary key in some other table.
Q54) What will GRANT option do?
A54) It will grant privileges to a list of one or more users. If the GRANT option is used in conjunction with the “PUBLIC”
option, then all users will be granted privileges. Also you can grant privileges by objects and types.
Q55) What does the term “grant privileges” mean?
A55) Grant privileges means giving access/authority to DB2 users.
Q56) What is an image copy?
A56) It is an exact reproduction of all or part of a tablespace. DB2 provides utility programs to make full-image copies (to
copy the entire tablespace) or incremental image copies to copy only those pages that have been modified since the last
image copy.
Q57) What is meant by an index?
A57) An index is a set of row identifiers (RIDs) or pointers that are logically ordered by the values of a column that has
been specified as being an index. Indexes provide faster access to data and can enforce uniqueness on the row in a
table.
Q58) What is an index key?
A58) It is a column or set of columns in a table used to determine the order of index entries.
Q59) What is a join?
A59) A join is a relational operation that allows retrieval of data from two or more tables based on matching columns
values.
Q60) What is meant by locking?
A60) Locking is a process that is used to ensure data integrity. It also prevents concurrent users from accessing inconsistent
data. The data (row) is locked until a commit is executed to release the updated data.
Q61) What is meant by null?
A61) This is a special value that indicates the absence of data in a column. This value is indicated by a negative value,
usually -1.
Q62) What is an object?
A62) An object is anything that is managed by DB2 (that is databases, table spaces, tables, views, indexes or synonyms), but
not the data itself.
Q63) Describe referential integrity?
A63) Referential integrity refers to a feature in DB2 that is used to ensure consistency of the data in the database.
Q64) Describe a primary key?
A64) A primary key is a key that is unique, non-null, and is part of the definition of a table. A table must have a primary key
to be defined as a parent.
Q65) How would you find out the total number of rows in a table? - GS
A65) Use SELECT COUNT(*) ...
Q66) How do you eliminate duplicate values in SELECT? - GS
A66) Use SELECT DISTINCT ...
Q67) How do you select a row using indexes? - GS
A67) Specify the indexed columns in the WHERE clause.
Q68) What are aggregate functions?
A68) Bulit-in mathematical functions for use in SELECT clause.
Q69) How do you find the maximum value in a column? - GS
A69) Use SELECT MAX(...
Q70) Can you use MAX on a CHAR column?
A70) YES.
Q71) My SQL statement SELECT AVG(SALARY) FROM EMP-TABLE yields inaccurate results. Why?
A71) Because SALARY is not declared to have Null’s and the employees for whom the salary is not known are also
counted.
Q72) How do you retrieve the first 5 characters of FIRSTNAME column of EMP table?
A72) SELECT SUBSTR(FIRSTNAME,1,5) FROM EMP;
Q73) How do you concatenate the FIRSTNAME and LASTNAME from EMP table to give a complete name?
A73) SELECT FIRSTNAME || ' ' || LASTNAME FROM EMP;
Q74) What is the use of VALUE function?
A74) Avoid negative SQLCODEs by handling nulls and zeroes in computations.
Substitute a numeric value for any nulls used in computation.
Q75) What is UNION,UNION ALL? - GS
A75) UNION eliminates duplicates
UNION ALL: retains duplicates
Both these are used to combine the results of different SELECT statements.
Q76) Suppose I have five SQL SELECT statements connected by UNION/UNION ALL, how many times should I
specify UNION to eliminate the duplicate rows? - GS
A76) Once.
Q77) What is the restriction on using UNION in embedded SQL?
A77) It has to be in a CURSOR.
Q78) In the WHERE clause what is BETWEEN and IN? - GS
A78) BETWEEN supplies a range of values while IN supplies a list of values.
Q79) Is BETWEEN inclusive of the range values specified? - GS
A79) Yes.
Q80) What is 'LIKE' used for in WHERE clause? What are the wildcard characters? - GS
A80) LIKE is used for partial string matches. '%' ( for a string of any character ) and '_' (for any single character ) are the
two wild card characters.
Q81) When do you use a LIKE statement?
A81) To do partial search e.g. to search employee by name, you need not specify the complete name; using LIKE, you can
search for partial string matches.
Q82) What is the meaning of underscore ( '_' ) in the LIKE statement? - GS
A82) Match for any single character.
Q83) What do you accomplish by GROUP BY ... HAVING clause? - GS
A83) GROUP BY partitions the selected rows on the distinct values of the column on which you group by. HAVING selects
GROUPs which match the criteria specified
Q84) Consider the employee table with column PROJECT nullable. How can you get a list of employees who are not
assigned to any project?
A84) SELECT EMPNO FROM EMP WHERE PROJECT IS NULL;
Q85) What is the result of this query if no rows are selected:
SELECT SUM(SALARY) FROM EMP WHERE QUAL='MSC';
A85) NULL
Q86) Why SELECT * is not preferred in embedded SQL programs?
For three reasons:
If the table structure is changed (a field is added), the program will have to be modified
Program might retrieve the columns which it might not use, leading on I/O over head.
The chance of an index only scan is lost.
Q87) What are correlated subqueries? - GS
A subquery in which the inner ( nested ) query refers back to the table in the outer query. Correlated
subqueries must be evaluated for each qualified row of the outer query that is referred to.
Q88) What is a cursor? Why should it be used? - GS
Cursor is a programming device that allows the SELECT to find a set of rows but return them one at a time.
Cursor should be used because the host language can deal with only one row at a time.
Q89) How would you retrieve rows from a DB2 table in embedded SQL? - GS
Either by using the single row SELECT statements,or by using the CURSOR.
Q90) Apart from cursor, what other ways are available to you to retrieve a row from a table in embedded SQL? - GS
Single row SELECTs.
Q91) How do you specify and use a cursor in a COBOL program? - GS
Use DECLARE CURSOR statement either in working storage or in procedure division (before open cursor),
to specify the SELECT statement. Then use OPEN, FETCH rows in a loop and finally CLOSE.
Q92) What happens when you say OPEN CURSOR?
If there is an ORDER BY clause, rows are fetched, sorted and made available for the FETCH statement. Other wise simply the cursor is placed on the first row.
Q93) Is DECLARE CURSOR executable?
No.
Q94) Can you have more than one cursor open at any one time in a program ? - GS
Yes.
Q95) When you COMMIT, is the cursor closed?
Yes.
1. What is SQLCA and SQLDA?
2. What is 2 phase commit?
No comments:
Post a Comment