155 lines
5.6 KiB
HTML
155 lines
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HTML
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<h1>Constraint Conflict Resolution in SQLite</h1>
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<p>
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In most SQL databases, if you have a <a href="lang_createtable.html#uniqueconst">UNIQUE</a>, <a href="lang_createtable.html#notnullconst">NOT NULL</a>, or
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<a href="lang_createtable.html#ckconst">CHECK</a> constraint on
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a table and you try to do an <a href="lang_update.html">UPDATE</a> or <a href="lang_insert.html">INSERT</a> that violates
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the constraint, the database will abort the operation in
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progress, back out any prior changes associated with the same
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UPDATE or INSERT statement, and return an error.
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This is the default behavior of SQLite, though SQLite also allows one to
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define alternative ways for dealing with constraint violations.
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This article describes those alternatives and how to use them.
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</p>
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<h2>Conflict Resolution Algorithms</h2>
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<p>
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SQLite defines five constraint conflict resolution algorithms
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as follows:
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</p>
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<dl>
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<dt><b>ROLLBACK</b></dt>
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<dd><p>When a constraint violation occurs, an immediate ROLLBACK
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occurs, thus ending the current transaction, and the command aborts
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with a return code of SQLITE_CONSTRAINT. If no transaction is
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active (other than the implied transaction that is created on every
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command) then this algorithm works the same as ABORT.</p></dd>
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<dt><b>ABORT</b></dt>
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<dd><p>When a constraint violation occurs, the command backs out
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any prior changes it might have made and aborts with a return code
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of SQLITE_CONSTRAINT. But no ROLLBACK is executed so changes
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from prior commands within the same transaction
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are preserved. This is the default behavior for SQLite.</p></dd>
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<dt><b>FAIL</b></dt>
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<dd><p>When a constraint violation occurs, the command aborts with a
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return code SQLITE_CONSTRAINT. But any changes to the database that
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the command made prior to encountering the constraint violation
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are preserved and are not backed out. For example, if an UPDATE
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statement encountered a constraint violation on the 100th row that
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it attempts to update, then the first 99 row changes are preserved
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but change to rows 100 and beyond never occur.</p></dd>
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<dt><b>IGNORE</b></dt>
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<dd><p>When a constraint violation occurs, the one row that contains
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the constraint violation is not inserted or changed. But the command
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continues executing normally. Other rows before and after the row that
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contained the constraint violation continue to be inserted or updated
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normally. No error is returned.</p></dd>
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<dt><b>REPLACE</b></dt>
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<dd><p>When a UNIQUE constraint violation occurs, the pre-existing row
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that caused the constraint violation is removed prior to inserting
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or updating the current row. Thus the insert or update always occurs.
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The command continues executing normally. No error is returned.</p></dd>
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</dl>
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