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sql.docx - A database index is a data structure that...

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Adatabase indexis adata structurethat improves the speed of data retrieval operations onadatabase tableat the cost of additional writes and storage space to maintain the index datastructure. Indexes are used to quickly locate data without having to search every row in a databasetable every time a database table is accessed.An index is a copy of selected columns of data from a table that can be searched very efficiently thatalso includes a low-level disk block address or direct link to the complete row of data it was copiedfrom.Indexes are used to policedatabase constraints, such as UNIQUE, EXCLUSION,PRIMARYKEYandFOREIGN KEY. Database systems usually implicitly create an index on a set of columnsdeclared PRIMARY KEY. Many database systems require that both referencing and referenced setsof columns in a FOREIGN KEY constraint are indexed, thus improving performance of inserts,updates and deletes to the tables participating in the constraint.Length/value:in case of numeric data types, it represents the display width (to which shorter valuesare padded in left; longer values are left as they are), not constraint.Foreign key dropdown: Phpmyadmin by default shows foreign key value and acustom descriptionfieldnext to it.The reason why it appears twice is to be able to type the first letter of either the key or thedisplay field.Take a backup:The export link and to restore click on the import link
Sql commands:CREATE DATABASEdatabasename;DROP DATABASEdatabasename;CREATETABLEtable_name(column1 datatype,column2 datatype,column3 datatype,....);CREATETABLEdatabase.table_name(column1 datatype,column2 datatype,column3 datatype,....);DROP TABLEtable_name;ALTERTABLEtable_nameADDcolumn_name datatype;ALTERTABLEtable_nameDROPCOLUMNcolumn_name;ALTERTABLEtable_nameALTERCOLUMNcolumn_name datatype;//change datatype or add a constraintSQL Constraints:Constraints can be specified when the table is created with the CREATE TABLEstatement, or after the table is created with the ALTER TABLE statement.CREATETABLEtable_name(column1 datatypeconstraint,column2 datatypeconstraint,column3 datatypeconstraint,....

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