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Print PDF-417 2d barcode in Font Figure 8-1. The results of attempting to violate check constraints

Figure 8-1. The results of attempting to violate check constraints
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CHAPTER 8 MOVING LOGIC TO THE DATABASE
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Adding UNIQUE Constraints
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You can specify that a column or columns in a table contain unique values. Unlike primary keys, which you will learn more about in the next section, unique columns may contain one NULL value. In the case of multiple columns, you must decide whether to have a constraint on each column or a constraint that covers several columns. In the first case, each column value must be unique; in the second case, the combination of the column values must be unique. You can add UNIQUE constraints to tables when you create them or later with an ALTER TABLE statement. Here is the syntax: --Adding individual constraints CREATE TABLE <table name> (<col1> <data type> UNIQUE, <col2> <data type> UNIQUE) --Adding a combination constraint CREATE TABLE <table name> (<col1> <data type>, <col2> <data type>, CONSTRAINT <constraint name> UNIQUE (<col1>,<col2)) --Add a constraint with ALTER TABLE CREATE TABLE <table name> (<col1> <data type>, <col2> <data type>) ALTER TABLE ADD CONSTRAINT <constraint name> UNIQUE (<col1>,<col2) The first syntax example creates a separate constraint on each column within the CREATE TABLE statement. The other two examples each create one constraint on a combination of the columns. If you do not specify a constraint name as in the first syntax example, SQL Server will come up with a name for you. Listing 8-2 contains example code showing how to create UNIQUE constraints. Type in and execute the code to learn more. Listing 8-2. Creating Tables with UNIQUE Constraints USE tempdb; GO --1 IF OBJECT_ID('table1') IS NOT NULL BEGIN DROP TABLE table1; END; --2 CREATE TABLE table1 (col1 INT UNIQUE, col2 VARCHAR(20), col3 DATETIME); GO --3 ALTER TABLE table1 ADD CONSTRAINT unq_table1_col2_col3 UNIQUE (col2,col3);
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CHAPTER 8 MOVING LOGIC TO THE DATABASE
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--4 PRINT 'Statement 4' INSERT INTO table1(col1,col2,col3) VALUES (1,2,'1/1/2009'),(2,2,'1/2/2009'); --5 PRINT 'Statement 5' INSERT INTO table1(col1,col2,col3) VALUES (3,2,'1/1/2009'); --6 PRINT 'Statement 6' INSERT INTO table1(col1,col2,col3) VALUES (1,2,'1/2/2009'); --7 PRINT 'Statement 7' UPDATE table1 SET col3 = '1/2/2009' WHERE col1 = 1; Figure 8-2 shows the results. Code section 1 drops the table in case it already exists. Statement 2 creates table1 with three columns. It creates a UNIQUE constraint on col1. Statement 3 adds another UNIQUE constraint on the combination of columns col2 and col3. Statement 4 adds two rows to the table successfully. Statement 5 violates the constraint on col2 and col3. Statement 6 violates the constraint on col1. Statement 7 violates the constraint on col2 and col3 with an UPDATE to the table.
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Figure 8-2. The results of adding UNIQUE constraints Another interesting thing about UNIQUE constraints is that you will not see them in the Constraints section in SQL Server Management Studio. Instead, you will find them in the Indexes
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section. When creating a unique constraint, you are actually creating a unique index. Figure 8-3 shows the constraints, as indexes, added to table1.
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Figure 8-3. The unique constraints defined on table1 are indexes.
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Adding a Primary Key to a Table
Throughout this book, you have read about primary keys. You can use a primary key to uniquely define a row in a table. A primary key must have these characteristics: A primary key may be made of one column or multiple columns, called a composite key. A table can have only one primary key. The values of a primary key must be unique. If the primary key is a composite key, the combination of the values must be unique. None of the columns making up a primary key can contain NULL values.
I once received a call from a developer asking me to remove the primary key from a table because it was preventing him from inserting rows into a table in one of our enterprise systems. He insisted that the table definition must be wrong. I spent ten minutes explaining that the primary key was preventing him from making a mistake and helped him figure out the correct statements. After this developer moved on to another company, I received almost the identical phone call from his replacement. Primary keys and other constraints are there to ensure data consistency, not to make your job harder. You can add a primary key to a table when you create the table using the CREATE TABLE statement or later by using the ALTER TABLE statement. Here is the syntax: --Single column key CREATE TABLE <table name> (<column1> <data type> NOT NULL PRIMARY KEY [CLUSTERED|NONCLUSTERED] <column2> <data type>) --Composit key CREATE TABLE <table name>(<column1> <data type> NOT NULL, <column2> <data type> NOT NULL, <column3> <data type>, CONSTRAINT <constraint name> PRIMARY KEY [CLUSTERED|NONCLUSTERED] (<column1>,<column2>) )
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