Constraints in Java

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As a programmer, you create database constraints so that garbage won't go in, and therefore less garbage will come out you hope Is garbage-free data itself an optimization Can it give the optimizer and the user better information to help in evaluating SELECTs Or will the constant error testing hinder INSERTs, UPDATEs, and DELETEs In this chapter, we'll address the good and bad effects of constraints on optimization Portability MySQL does not support triggers or constraints (unless they're implicit NOT NULL constraints or explicit PRIMARY KEY and UNIQUE constraints) The gains shown throughout this chapter are for only seven DBMSs unless the test specifically uses only those three constraint types
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NOT NULL
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The NULL "value" is a concept required by relational theory's data integrity rules That is, relational theory recognizes that, in some situations, a datum is not available, not known, or inapplicable NULL is used to represent all three of these cases It's important to remember that NULL does not equate to a blank or a zero: it's something else entirely Though a blank is equal to another blank and a zero is equal to another zero, a NULL is never equal to anything, not even another NULL NOT NULL constraints are used to enforce a rule that a column must always contain valid and applicable data in short, that a column may not contain NULL In the SQL Standard, this syntax:
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<column>NOT NULL
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is just a shorthand form for:
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CHECK (<column> IS NOT NULL)
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That is, NOT NULL constraints are just CHECK constraints according to the SQL Standard With most DBMSs, though, NOT NULL constraints are effectively quite different from ordinary CHECK constraints in these ways:
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NOT NULL constraints are usually unnamed and can't be dropped Defining a column as NOT NULL has an effect on the way that the column values are stored NOT NULL constraints are sometimes compulsory
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NOT NULL affects storage because it is impossible to store a NULL value in the column itself For instance, if a column is defined as SMALLINT, then it may contain any negative or positive number that fits in 16 bits but there is no room for a reserved 16-bit value that could mean NULL Therefore the DBMS must have a flag outside the column itself that says either "this is NULL" or "this is not NULL" In Microsoft, this flag is part of a bit list at the start of the row In IBM and most other DBMSs, the flag is a byte at the start of the column Either way, a column that can contain NULL takes more space to store than does a NOT NULL column A column that can contain NULL also takes slightly longer to retrieve, because you must always add a check for a NULL indicator to your program before processing the value itself The severest effect is seen with Sybase, because Sybase silently changes fixed-size columns (such as CHAR) into variable-
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length columns (such as VARCHAR) if they can contain NULL The result is that with unindexed columns, most DBMSs handle columns that can contain NULL slower than NOT NULL columns On the other hand, with indexed columns, Oracle handles nullable columns faster Some DBMSs have one or more gotchas that make columns that can contain NULL unwieldy For example:
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They make the assumption that all column definitions imply NOT NULL They require that primary keys be explicitly declared NOT NULL[1]
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Early versions of MySQL required that all columns in any index must be defined as NOT NULL This defect does not exist in MySQL version 323
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They refuse to allow multiple occurrences of NULL when a column is defined as UNIQUE Some DBMSs have conflicting rules in this area because they treat a UNIQUE constraint differently from a UNIQUE INDEX key Of the Big Eight, only IBM does this With IBM, a UNIQUE constraint column must be defined as NOT NULL and therefore disallows any NULL values A column that is part of a UNIQUE INDEX key, though, is allowed one NULL value They do not allow NOT NULL constraints to be dropped This is a tricky area because, although the SQL Standard doesn't distinguish between NOT NULL in a column definition and a NOT NULL constraint, some DBMSs do The difference lies in distinguishing between NOT NULL as a column attribute that cannot be changed and CHECK (<column> IS NOT NULL) as a constraint that can be dropped The SQL Standard considers the two to be the same That is, with SQL:1999, these two definitions both result in a named constraint that can be dropped:
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Case #1: CREATE TABLE Table1 ( column1 INTEGER NOT NULL) Case #2: CREATE TABLE Table1 ( column1 INTEGER, CHECK (column1 IS NOT NULL))
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IBM, Ingres, and InterBase consider Case #1 and Case #2 to be different For these DBMSs, the first CREATE TABLE statement results in a column (column1) with a NOT NULL attribute, and this attribute may not be changed a clear violation of the SQL Standard These DBMSs also support the second CREATE TABLE statement, which results in a named constraint that can be dropped as the SQL Standard requires Informix, Microsoft, Oracle, and Sybase correctly conform to the SQL Standard for both Case #1 and Case #2 MySQL is SQL Standard-compliant for Case #1 that is, MySQL lets you use the ALTER TABLE statement to change a nullable column to NOT NULL and vice versa However, MySQL doesn't support named CHECK constraints and so is not SQL Standard-compliant for Case #2
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All four of these gotchas are flat-out violations of SQL:1999, but we can't do anything about that Until these "rogue DBMSs" conform to the SQL Standard, portable programs should contain NOT NULL declarations in all or most column definitions Table 10-1 shows which of the Big Eight treat NULL in a nonconformant fashion
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Notes on Table 10-1:
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Auto NOT NULL column This column is "Yes" if the DBMS assumes that column1 in this table definition is a NOT NULL column rather than a "NULL allowed" column as required by the SQL Standard:
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CREATE TABLE Table1 ( column1 INTEGER)
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Auto NOT NULL No No No No No Yes No No Yes Table 10-1 ANSI/DBMSs and NULL Gotchas Drop Multiple Force NOT NULL Force NOT NULL NULL NULL PKEY Column PKEY Table Yes Many No No No Yes No No Yes Yes Yes Yes One One Zero Zero One Many Many One Yes No Yes Yes No No No No Yes No Yes Yes No Yes No Yes