Oracle XSQL- P10 potx

Table 8.15 Set Comparison Operators OPERATOR DESCRIPTION EXAMPLE IN Tests if an operand SELECT ename FROM emp belongs to the specified set. WHERE sal IN (800,950,1100) NOT IN Tests if an operand doesn't SELECT ename FROM emp belong to the specified set. WHERE sal NOT IN (800,950,1100) ANY Used in conjunction with SELECT ename FROM emp a relationship comparison WHERE sal > ANY operator. Determines if the (800,950,1100) specified relationship is true for any of the values. SOME Used in conjunction with SELECT ename FROM emp a relationship comparison WHERE sal > SOME operator. Determines if the (800,950,1100) specified relationship is true for one or more of the values. ALL Used in conjunction with SELECT ename FROM emp a relationship comparison WHERE sal > ALL operator. Determines if the (800,950,1100) specified relationship is true for all of the values. The Imaginary Dual Table Oracle provides the dual table, an imaginary table used largely for allowing you to per- form functions. For instance, if you want to use a select statement to get the current date, you could do so as follows: SELECT sysdate FROM dual Table 8.16 Set Operators OPERATOR DESCRIPTION UNION All unique rows of both queries are returned. UNION ALL All rows of both queries are returned. MINUS Eliminates rows that appear in the second query from the rows returned in the first query. INTERSECT Returns only the rows that are common to both queries. 160 Chapter 8 Of course, to do this from XSQL requires that you perform the following: <?xml version=”1.0”?> <page connection=”demo” xmlns:xsql=”urn:oracle-xsql”> <xsql:query> SELECT sysdate AS “Date” FROM dual </xsql:query> </page> You can also use the dual table to include parameters in the result set: <?xml version=”1.0”?> <page connection=”demo” xmlns:xsql=”urn:oracle-xsql”> <xsql:query> select ‘{@param}’ AS “ParamName” FROM dual </xsql:query> </page> Managing Tables The select statement is the tool used for getting data out of the database. The flip side of select is getting data in to the database. Before you can do that, however, you must have places to put the data. The construction of the database is the job of Data Definition Language (DDL) statements, which create, modify, or delete objects in the Oracle database. This section covers the part of DDL that pertains to the most popular and useful type of object, tables. There are a lot of things to consider when managing your table. At the highest level, you must decide what data you want in the table. You think about this based on how you want this table to fit in to the rest of your application. But there are lots of system level attributes to consider, also. Ultimately, you’ll want to work with your DBA on a lot of these parameters. Here, you’ll get a gentle introduction. Creating Tables There are many options in creating tables. In this section, first you’ll examine the sim- plest way to create a table; then you’ll examine some of the more useful aspects of table creation. A lot of the options presented here concern how the table is stored. Finally, you’ll learn how to create an exact copy of another table. The following SQL will create a table for customer orders. You’ll use this table in later examples, so you should create it under the momnpop id. CREATE TABLE customer ( custid NUMBER(8), fname VARCHAR2(30), lname VARCHAR2(30), address1 VARCHAR2(50), Oracle SQL 161 address2 VARCHAR2(50), state VARCHAR(5), zip VARCHAR(10), country VARCHAR(5) ); If you now do a desc customer, you’ll see that your table is in place. There are, however, a few problems that you will run into immediately. First, the custid will be used to identify your customers uniquely. You don’t want someone using the same cus- tomer identification (id) twice. There is an easy way to prevent this from happening— just issue the following command: TIP To get rid of the old table, just issue the command DROP TABLE customer; CREATE TABLE customer ( custid NUMBER(4) PRIMARY KEY, fname VARCHAR2(30), lname VARCHAR2(30), address1 VARCHAR2(50), address2 VARCHAR2(50), state VARCHAR(5), zip VARCHAR(10), country VARCHAR(5) ); The database will now automatically prevent anyone from reusing the same custid. Your next problem concerns the country column. Because most of your cus- tomers are in the United States, you will want to set USA as the default value. If the value of the country column isn’t explicitly defined in an insert statement, it will be USA. The first sentence asks that you set USA as the default value; the second sentence (as well as first sentence following the display code below) says that if you don’t explicitly define (set?) the default value, it will be USA. You can define the default value with the following code: CREATE TABLE customer ( custid NUMBER(4) PRIMARY KEY, fname VARCHAR2(30), lname VARCHAR2(30), email VARCHAR2(30), address1 VARCHAR2(50), address2 VARCHAR2(50), state VARCHAR(5), zip VARCHAR(10), country VARCHAR(5) DEFAULT ‘USA’ ); 162 Chapter 8 Now, one thing you do want your user to define is his or her last name. If you don’t define the last name, the row will be completely useless. You can require that the last name be defined as follows: CREATE TABLE customer ( custid NUMBER(4) PRIMARY KEY, fname VARCHAR2(30), lname VARCHAR2(30) NOT NULL, email VARCHAR2(30), address1 VARCHAR2(50), address2 VARCHAR2(50), state VARCHAR(5), zip VARCHAR(10), country VARCHAR(5) DEFAULT ‘USA’ ); NOTE The NOT NULL and PRIMARY KEY are examples of constraints, which you will learn more about later in the chapter. The best time to create them is along with the table, so a couple are introduced here. You’ll see all of them in a little while. Your next problem is that you want the table to live on a particular tablespace. You expect to get a lot of customers, so you want this table to be on your new terabyte drive. Here’s how you would do that, assuming the tablespace is named cust_tablespace: CREATE TABLE customer ( custid NUMBER(4) PRIMARY KEY, fname VARCHAR2(30), lname VARCHAR2(30) NOT NULL, email VARCHAR2(30), address1 VARCHAR2(50), address2 VARCHAR2(50), state VARCHAR(5), zip VARCHAR(10), country VARCHAR(5) DEFAULT ‘USA’ ) TABLESPACE cust_tablespace; NOTE There are many other storage options for your table that allow you to fine-tune how your data is stored. They require a deep understanding of the Oracle architecture to be used properly. These options are beyond the scope of this text and aren’t covered here. Oracle SQL 163 Let’s say that you want to base your table on data that already exists in the system. By using the AS keyword, you can create a new table based on the following select statement, and the table will automatically populate. Notice that you use the aliasing you learned earlier to change the empno column name to EMPID. CREATE TABLE empdept AS SELECT empnoAS”EMPID”,ename,dname FROM emp,dept WHERE emp.deptno=dept.deptno; The last topic for this section concerns temporary tables, used for storing data on a per-session basis. The data you put in can be seen only by your session, at the end of which the data goes away. Other sessions can use the same temporary table at the same time and your session won’t see their data. Temporary tables are often used in complex queries for which you need to grab a subset of data. They have limited usefulness in conjunction with XSQL pages because of the short duration of an XSQL session. How- ever, there may be some situations in which you’ll find it much easier and efficient to process a subset from a temporary table rather than to repeatedly requery the database or load the data set into memory. Here is how a temporary table is created: CREATE GLOBAL TEMPORARY TABLE temptable ( temp_col1 VARCHAR2(20), temp_col2 VARCHAR2(20) ); Altering Tables The alter table statement allows you to change aspects in a table that has already been created. In many cases, you can make changes to a table that already has data in it. This section looks at how the alter table statement works, what it is usually used for, and what it can’t be used for. Before beginning the discussion, it’s important to note that you can modify the storage characteristics of tables. These characteristics won’t be itemized here, but most can be altered at any time. In this section, most of the empha- sis is on working with columns, but it also covers moving a table to a new tablespace. Some discussion of constraints is given, but the “Constraints” section provides the greatest discussion of that topic. Working with Columns The alter table statement works almost exactly like the create table statement; it shares most of the same keywords and syntax. However, to use the alter table statement with columns is troublesome when you work with existing columns that are already populated with data. It is far simpler to add a column to a table. Here’s an example that uses the emp table you created previously. It adds to the table a home_phone column. 164 Chapter 8 ALTER TABLE emp ADD (home_phone VARCHAR2(10)); The good news is that adding a basic column is easy. It’s even easy to add a column that has a default value, as follows: ALTER TABLE emp ADD (benefits VARCHAR2(20) DEFAULT ‘STANDARD HEALTH’); But if your table already has data in it, things get more complex. For instance, what if you want to create a column as NOT NULL? Each row has to have a value. As shown in the following, the syntax is simple enough: ALTER TABLE emp ADD (home_zip VARCHAR2(10) NOT NULL); If you run this against the emp table, which has data in it, you will get the following error message: ORA-01758: table must be empty to add mandatory (NOT NULL) column The problem is that at the time you add the column, all the data is null. The NOT NULL constraint couldn’t possibly be satisfied. You have several alternatives: ■■ Set a default value for the column, such as ALTER TABLE emp ADD (home_zip VARCHAR2(10) NOT NULL DEFAULT ‘27607’); ■■ Remove all the data and alter the column ■■ Create the column without the NOT NULL constraint, add the data, and apply the NOT NULL constraint afterwards. The second option leads directly to the discussion about altering existing columns. To make these alterations, you would use the modify keyword and a similar expres- sion that you would use if you were creating the column from scratch. ALTER TABLE emp ADD ( home_zip VARCHAR2(10)); . . . add the data home zip data. ALTER TABLE emp MODIFY ( home_zip NOT NULL); You don’t need to include anything about the data type, since that would remain the same. However, if you want to change the data type, you can. But there are restrictions. Anything goes if the column has no data in it. If you want, you can even change the data type entirely from varchar2 to date, date to number, number to varchar2, and so on. If, however, there is any data in the column, this kind of modification isn’t allowed. Table 8.17 lists the rules for modifying columns that already contain data. Oracle SQL 165 Table 8.17 Modifying Columns Containing Data ACTION DESCRIPTION OKAY? EXAMPLE Assigning Using the DEFAULT Always ALTER TABLE emp default value keyword to specify a MODIFY (home_zip default value for the DEFAULT 27609) column. The default value will only be used moving forward; null and other values already in the system will remain the same. Widening strings Increasing the number Always ALTER TABLE emp of characters allowed. MODIFY (home_zip VARCHAR2(12)) Narrowing strings Decreasing the Only to ALTER TABLE emp number of characters longest MODIFY (ename allowed. Oracle won't string VARCHAR2(7)); do this if it would length require any existing string to be shortened. Increasing scale Increasing the number Always ALTER TABLE emp For numbers of digits to the left MODIFY (empno of the decimal point. NUMBER(6,0)) Increasing Increasing the number Always ALTER TABLE emp precision For of digits to the right MODIFY (empno numbers of the decimal point. NUMBER(8,2)) Decreasing scale Shrinking the size of No and precision a number column on For numbers either the left or right of the column. Our last topic for this section concerns dropping a column altogether, the ultimate modification. This can be accomplished by way of the drop column clause. If there is data in the column, it will be deleted. alter table emp drop column home_zip; Dropping Tables Dropping tables is the process of removing a table and all of its data from your data- base. To drop a table, you need the drop table privilege for the table. Also, you need to be very careful; if you are having a bad day, you might end up inadvertently drop- ping the incorrect table. 166 Chapter 8 There are two ways to drop tables. The following way you’ve already seen: DROP TABLE table_to_drop; Often when trying to drop a table, you will encounter the following error message: ORA-02449: unique/primary keys in table referenced by foreign keys You get this error message because foreign key constraints are being used. You learn more about foreign key constraints later; for now, it’s important to know only that there are rows in other tables that reference keys in this table. If you just go and drop this table, those other tables will be very, very upset. Thus, Oracle doesn’t allow you to do it. Here are ways around this problem: ■■ Getting rid of the foreign key constraints; then dropping the table ■■ Deleting the data that references this table ■■ Dropping this table and all the tables with references to it The first two options will be examined in the foreign constraints discussion. The last option is a bit draconian and risky: You delete the table_to_drop table and recur- sively drop all tables that have a reference to it. Oracle will drop the tables with refer- ences to your target table, and if it finds that it can’t drop a table because another table has a reference to it, it will go and find that table and then drop that one too. It’s like a scorched-earth campaign against the ORA-02449 error. Use with care. DROP TABLE table_to_drop CASCADE CONSTRAINTS; Adding and Modifying Data Now that you’ve created some tables, you are ready to put data in them. Data is added to tables by way of the insert statement, existing data is changed by way of the update statement, and data is removed by way of the delete statement. The cus- tomer table created earlier for examples is used for discussing all these statements. Before these statements are discussed, sequences are introduced. Sequences solve a fundamental problem—that of creating unique keys for your tables. But before you read about any of these topics, you need to learn about Oracle transactions. When you add and modify data, you use the xsql:dml action. This action isn’t lim- ited to only one SQL statement the way that the xsql:select action is. Even though our examples here use one statement only, you can combine as many statements as you like between the begin and end statements inside the xsql:dml element. Transactions A transaction is one or more SQL statements executing against the database. They are very important when you work with Data Manipulation Language (DML) statements because you often want all of the statements to succeed or fail as a group. A classic Oracle SQL 167 example is a banking transaction in which $100 is transferred from a savings to a checking account. This transaction really involves three steps: 1. Subtract $100 from savings. 2. Add $100 to checking. 3. Record the transaction in the transaction log. If any of the preceding statements fail individually, you will have problems. If the cus- tomer doesn’t have $100 in savings, you will hand him or her free money to put into their checking account. If a problem occurs in adding the money to checking, you will have an upset customer. If you can’t log the transaction, you will have bookkeeping problems in the future. Oracle addresses this problem with the commit, rollback, and savepoint state- ments. No data is saved permanently to the database until a commit is issued. The commit is the end of the transaction, and the first SQL statement issued—either in a session or after a commit—is the beginning of the session. If you run into trouble halfway through a transaction, you can issue a rollback to tell Oracle to ignore all the statements that have been executed so far in the transaction. The database is rolled back to the state that it was in before the start of the transaction. You can also specify a savepoint, which is an intermediate commit. At the time you issue a savepoint, no data is permanently written to the database; instead, you have a point to roll back to other than the beginning of the transaction. This can save time and horsepower, because you don’t have to repeat all of the work that was done before you issue the savepoint, which is presumably okay. Sequences Earlier, you created a table with a primary key. The purpose of the primary key is to uniquely identify each row in your table. Each time you insert a new row, you need to create a new, unique key. You know that Oracle won’t let you insert a nonunique key, but how do you generate a new key? Finding the last key used is one strategy, but what if multiple sessions are creating new keys at the same time? Fortunately, there is an easy solution—a sequence. An Oracle sequence generates a sequence of numbers. The numbers are guaranteed to be unique. Each time a call is made, the next number in the sequence is returned. Here is a simple sequence: create sequence my_seq; To use the sequence to generate ids, you can use the dual table as follows: <?xml version=”1.0”?> <page connection=”demo” xmlns:xsql=”urn:oracle-xsql”> <xsql:query> 168 Chapter 8 SELECT my_seq.nextval FROM dual </xsql:query> </page> If you are interested in getting the last value that was selected—known as the current value—you can do that as follows: SELECT my_seq.currval FROM dual Each time you reload the page, you will get a new sequence number. By default, a sequence starts at 1, increments by 1, has no max value (other than 10^27), will never cycle back to where it started, will cache 20 numbers at a time, and does not guarantee that numbers will be generated in the order of the request. All these options can be tweaked, as listed in Table 8.18. Table 8.18 Sequence Options KEYWORD DESCRIPTION EXAMPLE INCREMENT BY Value to increment by create sequence between calls. Can be negative. my_seq increment by 2 START WITH The first sequence number to create sequence be generated. Defaults to my_seq start with 10 MINVALUE for ascending sequences and MAXVALUE for descending sequences. MAXVALUE The largest value possible for create sequence the sequence. Default is 10^27. my_seq MAXVALUE 9999 MINVALUE The smallest value possible create sequence for the sequence. Default is 1. my_seq MINVALUE 20 CYCLE The sequence will cycle when create sequence either MINVALUE or MAXVALUE my_seq CYCLE is reached. CACHE Number of numbers that will Create sequence be kept in cache. Unused my_seq CACHE 100 numbers are lost when the database shuts down. ORDER The sequence will guarantee CREATE SEQUENCE that numbers are returned in my_seq ORDER the order that requests were received. Oracle SQL 169 [...]... specifics of date formatting, it discusses the differences between SQL*PLUS and XSQL XSQL Dates versus Oracle Dates SQL*PLUS uses the default date format of the database, while XSQL uses the default date format of the JDBC driver Thus, executing this statement from SQL*PLUS, SELECT sysdate AS cur_date FROM dual Oracle SQL will probably result in a date like this one: 18.May-02 The following XSQL, which uses... can explicitly specify the partition or subpartition of your table into which you wish to insert the data Except for a couple of PL/SQL specific options that you will see later in the book, that is it Oracle SQL N OT E In your travels, you may have seen an Insert statement without the columns specified This only works when you are inserting a value into every column of the table In general, this is... job=’CLERK’; The real danger in the delete statement is getting the where clause wrong or accidentally forgetting it The following delete statement gets rid of all the rows in your table: DELETE FROM emp; Oracle also gives you the truncate statement to get rid of all the data in your table The truncate statement deletes all the data quickly—there is no way to roll back a truncate Only use when you know... Views Creating a view is almost as simple as writing a select statement Here is an example where you create a view that pulls the employee name and department name together: CREATE VIEW emp_dept_name AS Oracle SQL SELECT ename,dname FROM emp,dept WHERE emp.deptno=dept.deptno; This view has two columns, ename and dname The options for altering a view are exactly the same as those for creating one Usually,... SET sal=1000; You may want to prevent such modifications To do so, use the WITH OPTION keyword as follows: CREATE OR REPLACE VIEW emp_view AS SELECT * FROM emp WHERE sal>1300 WITH CHECK OPTION; CHECK Oracle SQL Constraints At this point, you are ready to be a pretty productive SQL coder You have learned how to read data out of the database, how to administer your objects, and how to manage the data... key and an on delete clause isn't specified for the foreign key constraint To force deletion of the parent row and all referencing children rows, you can use the cascade constraints option of delete Oracle SQL As you can see from Table 8.21, nulls are allowed on either side of a foreign key constraint Generally, though, a NOT NULL constraint is applied on the columns comprising both the foreign keys... figure out the name that was created for the constraint For primary and unique constraints, you can remove a constraint either with the word primary or based on the constraint’s definition Date Formatting Oracle stores dates in an internal format that isn’t very useful to you By default, dates are converted to a character representation that uses the default date format on the application that makes the... You specify the table, the columns you wish to insert, and the data Here is an example where you insert a new customer order from XSQL: BEGIN INSERT INTO customer (custid,lname,email) VALUES (my_seq.nextval,’SMITH’,’smith@ibiblio.org’); commit; END; You specify the columns that you wish to insert,... SQL will probably result in a date like this one: 18.May-02 The following XSQL, which uses the same SQL query, returns a different result SELECT sysdate AS “Date” FROM dual The result is shown in Figure 8.7 If you specify your own date using the to_char function, then you’ll get the same date . They require a deep understanding of the Oracle architecture to be used properly. These options are beyond the scope of this text and aren’t covered here. Oracle SQL 163 Let’s say that you want. you perform the following: <?xml version=”1.0”?> <page connection=”demo” xmlns:xsql=”urn :oracle- xsql”> <xsql:query> SELECT sysdate AS “Date” FROM dual </xsql:query> </page> You. parameters in the result set: <?xml version=”1.0”?> <page connection=”demo” xmlns:xsql=”urn :oracle- xsql”> <xsql:query> select ‘{@param}’ AS “ParamName” FROM dual </xsql:query> </page> Managing