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Effects of NULL for UNIQUE Specification
When a UNIQUE specification u for base table t includes a column c that is not subject to a NOT NULL constraint, the appearance of several rows having NULL in place of a value for c and equal values for the other columns specified in u is permitted. It is only when each column of the specified "key" has a value that those column values may not appear in the same combination in more than one row of t.
Transactions in SQL BEGIN TRANSACTION, COMMIT, and ROLLBACK, SQL has the same syntax except for START in place of BEGIN. However, START TRANSACTION is used only for outermost
SQLs counterpart of the key words: The text from the opening parenthesis to the end of the fourth line specifies the declared type of the table, meaning that every table ever
Example of DELETE - SQL As with UPDATE, a FOR PORTION OF clause can be specified if the target table has a defined period name, as illustrated in Example. Example: Deleting
Assignments in pl/sql The Variables and constants are initialized every time a block or subprogram is entered. By default, the variables are initialized to NULL. Therefore, unle
Explicitly specifying the join condition - SQL SELECT * FROM IS_CALLED JOIN IS_ENROLLED_ON ON ( IS_CALLED.StudentId = IS_ENROLLED_ON.StudentId ) Now, the key word JO
Using a Join on 2 tables, select all columns and all rows from the tables without the use of a Cartesian product. Query: SELECT * FROM EMPLOYEE1 JOIN CONTRACT ON EMPLOYEE
UPDATE Command- SQL Loosely speaking, UPDATE changes some of the column values of some existing rows of its target table. Thus, although some rows disappear from the target an
Pl/SQL Expressions The Expressions are constructed by using the operands and operators. An operand is a constant, literal, variable, or function call which contributes a value
Providing Results of Queries Expressing queries in SQL is the (big) subject. Here I present just a simple example to give you the flavour of things to come in those chapters.
Advantages of Subprograms The Subprograms give extensibility; that is, tailor the PL/SQL language to suit your requirements. For illustration, if you require a procedure which
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