SQL UNIQUE Constraint - W3Schools

Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has a ... 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ALTERTABLE AND ANY AS ASC BACKUPDATABASE BETWEEN CASE CHECK COLUMN CONSTRAINT CREATE CREATEDATABASE CREATEINDEX CREATEORREPLACEVIEW CREATETABLE CREATEPROCEDURE CREATEUNIQUEINDEX CREATEVIEW DATABASE DEFAULT DELETE DESC DISTINCT DROP DROPCOLUMN DROPCONSTRAINT DROPDATABASE DROPDEFAULT DROPINDEX DROPTABLE DROPVIEW EXEC EXISTS FOREIGNKEY FROM FULLOUTERJOIN GROUPBY HAVING IN INDEX INNERJOIN INSERTINTO INSERTINTOSELECT ISNULL ISNOTNULL JOIN LEFTJOIN LIKE LIMIT NOT NOTNULL OR ORDERBY OUTERJOIN PRIMARYKEY PROCEDURE RIGHTJOIN ROWNUM SELECT SELECTDISTINCT SELECTINTO SELECTTOP SET TABLE TOP TRUNCATETABLE UNION UNIONALL UNIQUE UPDATE VALUES VIEW WHERE MySQLFunctions StringFunctions: ASCII CHAR_LENGTH CHARACTER_LENGTH CONCAT CONCAT_WS FIELD FIND_IN_SET FORMAT INSERT INSTR LCASE LEFT LENGTH LOCATE LOWER LPAD LTRIM MID POSITION REPEAT REPLACE REVERSE RIGHT RPAD RTRIM SPACE STRCMP SUBSTR SUBSTRING SUBSTRING_INDEX TRIM UCASE UPPER NumericFunctions: ABS ACOS ASIN ATAN ATAN2 AVG CEIL CEILING COS COT COUNT DEGREES DIV EXP FLOOR GREATEST LEAST LN LOG LOG10 LOG2 MAX MIN MOD PI POW POWER RADIANS RAND ROUND SIGN SIN SQRT SUM TAN TRUNCATE DateFunctions: ADDDATE ADDTIME CURDATE CURRENT_DATE CURRENT_TIME CURRENT_TIMESTAMP CURTIME DATE DATEDIFF DATE_ADD DATE_FORMAT DATE_SUB DAY DAYNAME DAYOFMONTH DAYOFWEEK DAYOFYEAR EXTRACT FROM_DAYS HOUR LAST_DAY LOCALTIME LOCALTIMESTAMP MAKEDATE MAKETIME MICROSECOND MINUTE MONTH MONTHNAME NOW PERIOD_ADD PERIOD_DIFF QUARTER SECOND SEC_TO_TIME STR_TO_DATE SUBDATE SUBTIME SYSDATE TIME TIME_FORMAT TIME_TO_SEC TIMEDIFF TIMESTAMP TO_DAYS WEEK WEEKDAY WEEKOFYEAR YEAR YEARWEEK AdvancedFunctions: BIN BINARY CASE CAST COALESCE CONNECTION_ID CONV CONVERT CURRENT_USER DATABASE IF IFNULL ISNULL LAST_INSERT_ID NULLIF SESSION_USER SYSTEM_USER USER VERSION SQLServerFunctions StringFunctions: ASCII CHAR CHARINDEX CONCAT Concatwith+ CONCAT_WS DATALENGTH DIFFERENCE FORMAT LEFT LEN LOWER LTRIM NCHAR PATINDEX QUOTENAME REPLACE REPLICATE REVERSE RIGHT RTRIM SOUNDEX SPACE STR STUFF 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SQLExamples SQLExamples SQLQuiz SQLExercises SQLCertificate SQLUNIQUEConstraint ❮Previous Next❯ SQLUNIQUEConstraint TheUNIQUEconstraintensuresthatallvaluesinacolumnaredifferent. BoththeUNIQUEand PRIMARYKEYconstraintsprovideaguaranteeforuniquenessforacolumnorsetofcolumns. APRIMARYKEYconstraintautomaticallyhasa UNIQUEconstraint. However,youcanhavemanyUNIQUEconstraintspertable,butonlyone PRIMARYKEYconstraintpertable. SQLUNIQUEConstraintonCREATETABLE ThefollowingSQLcreatesaUNIQUEconstraintonthe"ID"columnwhenthe"Persons"tableiscreated: SQLServer/Oracle/MSAccess: CREATETABLEPersons (    IDintNOTNULLUNIQUE,     LastNamevarchar(255)NOTNULL,     FirstNamevarchar(255),     Ageint ); MySQL: CREATETABLEPersons (    IDintNOTNULL,     LastNamevarchar(255)NOTNULL,     FirstNamevarchar(255),     Ageint,     UNIQUE(ID) ); TonameaUNIQUEconstraint,andtodefinea UNIQUEconstraintonmultiplecolumns,usethefollowingSQLsyntax: MySQL/SQLServer/Oracle/MSAccess: CREATETABLEPersons (    IDintNOTNULL,     LastNamevarchar(255)NOTNULL,     FirstNamevarchar(255),     Ageint,     CONSTRAINTUC_PersonUNIQUE(ID,LastName) ); SQLUNIQUEConstraintonALTERTABLE TocreateaUNIQUEconstraintonthe"ID"columnwhenthetableisalreadycreated,usethefollowingSQL: MySQL/SQLServer/Oracle/MSAccess: ALTERTABLEPersons ADDUNIQUE(ID); TonameaUNIQUEconstraint,andtodefinea UNIQUEconstraintonmultiplecolumns,usethefollowingSQLsyntax: MySQL/SQLServer/Oracle/MSAccess: ALTERTABLEPersons ADDCONSTRAINTUC_PersonUNIQUE(ID,LastName); DROPaUNIQUEConstraint TodropaUNIQUEconstraint,usethefollowingSQL: MySQL: ALTERTABLEPersons DROPINDEXUC_Person; SQLServer/Oracle/MSAccess: ALTERTABLEPersons DROPCONSTRAINTUC_Person; ❮Previous Next❯ NEW WejustlaunchedW3Schoolsvideos Explorenow COLORPICKER Getcertifiedbycompletingacoursetoday! w3schoolsCERTIFIED.2022 Getstarted CODEGAME PlayGame