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Just as SQL data definition commands allow you to change the structure of a database, SQL data manipulation commands allow you to change the contents of your database. For this purpose, SQL offers three basic data manipulation commands: INSERT, to add rows to a table UPDATE, to change column values of existing rows DELETE, to remove rows from a table
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You can add rows to a table with the INSERT command in two ways. One way is to add rows one by one by specifying a list of column values in the VALUES clause of the INSERT statement. The other is to add one or more rows to a table based on a selection (and manipulation) of existing data in the database (called a subquery).
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Note You can also load data into an Oracle database with various tools specifically developed for this purpose such as Data Pump in Oracle Database 10g, Export and Import in previous Oracle releases, and SQL*Loader. These tools are often used for high-volume data loads.
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INTRODUCTION TO SQL, SQL*PLUS, AND SQL DEVELOPER
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Data manipulation commands are always treated as being part of a transaction. This means (among other things) that all database changes caused by SQL data manipulation commands get a pending status, until you confirm (commit) or cancel (roll back) the transaction. No one (except the transaction itself) can see the pending changes of a transaction before it is committed. That s why a transaction is often labeled atomic: it is impossible for other database users to see parts of a transaction in the database. It is all or nothing, no matter how many DML operations the transaction comprises. SQL offers two commands to control your transactions explicitly: COMMIT, to confirm all pending changes of the current transaction ROLLBACK, to cancel all pending changes and restore the original situation
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Sometimes, transactions are committed implicitly; that is, without any explicit request from a user. For example, every data definition command implicitly commits your current transaction. Note the following important differences between data manipulation and data definition: DELETE empties a table; DROP removes a table. TRUNCATE allows you to delete all the rows in a table in an efficient (but irrevocable) way. UPDATE changes the contents of a table; ALTER changes its structure. You can undo the consequences of data manipulation with ROLLBACK; data definition commands are irrevocable.
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6 will revisit data manipulation in more detail. 7 discusses the TRUNCATE command, which is considered a data definition command.
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The only SQL command used to query database data is SELECT. This command acts at the set (or table) level, and always produces a set (or table) as its result. If a certain query returns exactly one row, or no rows at all, the result is still a set: a table with one row or the empty table, respectively. The SELECT command (as defined in the ANSI/ISO SQL standard) has six main components, which implement all SQL retrieval. Figure 2-1 shows a diagram with these six main components of the SELECT command.
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Figure 2-1. The six main components of the SELECT command
INTRODUCTION TO SQL, SQL*PLUS, AND SQL DEVELOPER
The lines in this diagram represent all possibilities of the SELECT command, like a railroad map. You can deduce the following three syntax rules from Figure 2-1: The order of these six command components is fixed. The SELECT and FROM components are mandatory. The remaining components (WHERE, GROUP BY, HAVING, and ORDER BY) are optional.
Table 2-1 gives a high-level description of the roles of these six components of the SELECT command. Table 2-1. The Six Main Components of the SELECT Command
Component
FROM WHERE GROUP BY HAVING SELECT ORDER BY
Description
Which table(s) is (are) needed for retrieval What is the condition to filter the rows How should the rows be grouped/aggregated What is the condition to filter the aggregated groups Which columns do you want to see in the result In which order do you want to see the resulting rows
Tip The order of the SELECT command components as displayed in Table 2-1 is also a good order to think about them when writing SQL statements. Notice that the SELECT clause is almost the last one.
Components of the SELECT command implement three of the relational operators introduced in 1 (Section 1.6) as follows: The SELECT component acts as the projection operator. The FROM component implements the join operator. The restriction operator corresponds to the WHERE component.
Now that we are on the subject of relational operators, note that the union, intersection, and difference (minus) operators are also implemented in SQL. You can use these three set operators to combine the results of multiple SELECT commands into a single result table, as illustrated in Figure 2-2. We will revisit these operators in 8.
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