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Drop NULL column This column is "No" if the DBMS doesn't allow you to drop a NOT NULL constraint as required by the SQL Standard
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Multiple NULL column This column is "Zero" if the DBMS doesn't allow any NULLs in a UNIQUE column; "One" if the DBMS allows only one NULL in a UNIQUE column; and "Many" if the DBMS allows multiple NULLs to be inserted into a UNIQUE column as required by the SQL Standard (Note: By "UNIQUE column," we mean a column that is part of a UNIQUE constraint or a column that is part of a UNIQUE INDEX key We do not mean a column that is part of a PRIMARY KEY constraint, though such columns are also constrained to contain only unique values The difference between the two is that the SQL Standard allows UNIQUE columns to contain NULLs but disallows any NULLs in PRIMARY KEYs)
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Force NOT NULL PKEY Column column This column is "Yes" if the DBMS requires an explicit NOT NULL constraint when you define a PRIMARY KEY column constraint That is, the DBMS requires this definition:
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CREATE TABLE Table1 ( column1 INTEGER NOT NULL PRIMARY KEY)
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rather than this definition as required by the SQL Standard:[2]
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We should note that Entry Level SQL-92 the lowest conformance level requires the explicit NOT NULL definition we're deploring here and in the next paragraph Because each of the DBMSs that force this syntax declares its SQL compliance level to be Entry Level SQL-92, it is not in violation of the outdated Standard But SQL-92 was superseded in 1999, and Core SQL:1999 specifies that NOT NULL is to be implied because non-nullability is inherent in the definition of a primary key just as uniqueness is
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CREATE TABLE Table1 ( column1 INTEGER PRIMARY KEY)
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Force NOT NULL PKEY Table column This column is "Yes" if the DBMS requires an explicit NOT NULL constraint when you define a PRIMARY KEY table constraint That is, the DBMS requires this definition:
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CREATE TABLE Table1 ( column1 INTEGER NOT NULL, CONSTRAINT Constraint1 PRIMARY KEY (column1))
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rather than this definition as required by the SQL Standard:
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CREATE TABLE Table1 ( column1 INTEGER, CONSTRAINT Constraint1 PRIMARY KEY (column1))
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NULL Indicators
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Even if you define every column in every table as NOT NULL, you still need to add NULL indicator checks to your programs because there's still a possibility that NULLs might be returned For example, these three SQL statements could return NULLs under some circumstances:
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SELECT MAX(column1) FROM Table1 SELECT Table1column1, Table2column1 FROM Table1 LEFT JOIN Table2 ON Table1column1 = Table2column1 SELECT SUM(column1) FROM Table1 WHERE column2 > 100 /* returns NULL if no column2 value is greater than 100 */
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The Bottom Line: NOT NULL Constraints
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To speed things up, define columns and domains with NOT NULL as often as possible Even if your definitions always include NOT NULL, make sure your program allows for the possibility of retrieved NULLs Add NULL indicator checks for every OUTER JOIN retrieval and wherever it's possible that a query might be searching an empty table
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CHECK
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A CHECK constraint is used to enforce a rule that a column may contain only data that falls within some specific criteria To segue from NOT NULL constraints to CHECK constraints, let's ask the nasty question Is it redundant to have both At first glance, the answer appears to be "Yes" For example, suppose you have a table defined like this:
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CREATE TABLE Table1 ( column1 INTEGER NOT NULL, CHECK (column1 < 10) )
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Obviously a value that "must be less than ten" must automatically be NOT NULL anyway, right Wrong One of SQL's little-known details is that a CHECK constraint doesn't have to be true It only has to be "other than false"[3] which means a NULL would be accepted by the CHECK constraint Therefore a NOT NULL constraint is not a redundancy in this example