St a t e m e nt s for Cr e a t in g a nd D r oppin g Tr igge r s in VB.NET

Paint DataMatrix in VB.NET St a t e m e nt s for Cr e a t in g a nd D r oppin g Tr igge r s

St a t e m e nt s for Cr e a t in g a nd D r oppin g Tr igge r s
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An array of T- SQL st at em ent s ex ist for creat ing and m anaging t riggers. Many of t hese st at em ent s parallel t hose for ot her dat abase obj ect s, but som e are special for t r iggers. ( For exam ple, y ou already r ead about t he sp_set t r iggerorder syst em st ored procedure.) You can m ak e new t r iggers wit h t he CREATE TRI GGER st at em ent . This single T- SQL st at em ent facilit at es t he cr eat ion of AFTER and I NSTEAD OF t riggers by t he inclusion of a k eyw ord phrase specify ing t he t r igger t ype. The DROP TRI GGER st at em ent works for eit her t ype of t rigger, but t her e is no I NFORMATI ON_SCHEMA v iew t hat displays t he t r iggers in a dat abase. The follow ing sam ples dem onst rat e an appr oach t o checking for t he exist ence of a t rigger based on t he sysobj ect s t able in a dat abase. I t is som et im es conv enient t o disable a t r igger ( for ex am ple, t o ent er new r ows int o a t able t hat conflict wit h t he logic in t he t r igger) . Use t he ALTER TABLE st at em ent w it h t he DI SABLE keyw ord follow ed by t he t r igger nam e t o disable an ex ist ing t r igger . To r est or e t he t r igger, specify t he ENABLE keyw ord follow ed by t he t rigger s nam e w it hin an ALTER TABLE st at em ent . The CREATE TRI GGER st at em ent is flex ible because a single t em plat e accom m odat es bot h AFTER and I NSTEAD OF t r iggers. I n addit ion, y ou can specify eit her t r igger t ype for any com binat ion of t he t hr ee possible event s t hat can fire it . The t r igger nam e follow ing t he CREATE TRI GGER k eyw ord phrase is a norm al SQL Ser ver ident ifier. To m ak e t r igger nam es ident ify t heir obj ect t ype, t his chapt er begins t r igger nam es w it h t he t rg pr efix . Designat e t he obj ect t o w hich a t rigger belongs in t he ON clause. Specify t he obj ect by following t he ON keyword wit h t he nam e of a t able or v iew . The next line is w her e t he CREATE TRI GGER st at em ent offers m uch of it s flex ibilit y. You can st art t he line w it h eit her t he AFTER k ey word or t he I NSTEAD OF k eyw ord phrase t o declare t he t rigger t ype. Because AFTER is t he default t rigger t ype, y ou don t need t o specify t he AFTER k eyw ord t o creat e an AFTER t rigger . The FOR clause specifies t he t ype of event s t hat w ill fir e an AFTER t rigger . The synt ax for t he I NSTEAD OF k eyw or d doesn t requir e t he FOR k eyword ( as is t he case for AFTER t r iggers) . The follow ing code t em plat e shows all t hr ee ev ent s. Howev er, you can designat e any t wo or j ust one ev ent . The ev ent nam es in t he FOR clause det er m ine what act ions fire a t r igger. The AS keyw or d m arks
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t he t ransit ion fr om t he t rigger declarat ions t o t he T- SQL code t hat a t r igger ex ecut es when it fir es. CREATE TRIGGER trigger_name ON tablename or viewname AFTER OR INSTEAD OF OR FOR INSERT, UPDATE, DELETE AS T-SQL statements for trigger CREATE TRI GGER w ill fail if you t ry t o cr eat e a new t rigger w it h a nam e for a prev iously exist ing t r igger . Ther e are a couple of workarounds t o t his pr oblem . First , you can m odify t he design of t he old t rigger w it h t he ALTER TRI GGER st at em ent . Second, you can condit ionally dr op t he old version of a t r igger. The synt ax for checking on t he ex ist ence of a previously exist ing t r igger is different from t hat for t ables, v iews, st ored procedur es, and UDFs. This is because t here is no I NFORMATI ON_SCHEMA v iew for list ing t r iggers. However, y ou can use t he nam e and t ype colum ns of t he sysobj ect s t able t o v er ify t he exist ence of a prev iously exist ing version of a t rigger in a dat abase. The sysobj ect s t able is a t able m aint ained by SQL Serv er t hat keeps t rack of t he obj ect s in a dat abase. I n t he follow ing t em plat e, cont r ol passes t o t he DROP TRI GGER st at em ent only if t he sysobj ect s t able cont ains a row w it h a nam e colum n value equal t o t r igger nam e and a t ype value equal t o TR. IF EXISTS (SELECT name FROM sysobjects WHERE name = triggername AND type = TR ) DROP TRIGGER triggername You can use t he sam e synt ax for ver ify ing t he exist ence of m any ot her dat abase obj ect s. Change t ype t o V for views and P for st or ed procedur es. Use FN, I F, and TF for scalar, inline, and m ult ist at em ent UDFs, r espect iv ely.
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