what is difference between index and unique index in oracle?
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Index: It is a schema object which is used to provide improved performance in the retrieval of rows from a table. Unique Index: Unique indexes guarantee that no two rows of a table have duplicate values in the key column (or columns).
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A Clustered index is a type of index in which table records are physically reordered to match the index. A Non-Clustered index is a special type of index in which logical order of index does not match physical stored order of the rows on disk.
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Primary Key: that is used for uniquely identified a perticular row of a database table . Primary INDEX: That is used for getting a index from a database table and that indexing is done on unique based.
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['SELECT DISTINCT returns only distinct (i.e. different) values.', 'SELECT DISTINCT eliminates duplicate records from the results.', 'DISTINCT can be used with aggregates: COUNT, AVG, MAX, etc.', 'DISTINCT operates on a single column. DISTINCT for multiple columns is not supported.']
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Both "indexes" and "indices" are acceptable plural forms of the word "index" or to refer to more than one index. Index is one of those rare words that have two different plurals in English. "Indices" is originally a Latin plural, while "Indexes" has taken the English way of making plurals, using –s or –es.
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SQL Anywhere uses physical and logical indexes. A physical index is the actual indexing structure as it is stored on disk. A logical index is a reference to a physical index. ... If one does not exist, the database server creates a new physical index and then points the logical index to it.
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The names of database objects need not be unique in a database. However, column names and index names must be unique within a table, and other object names must be unique for each owner within a database.
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Sy-tabix is used to find the current line in the internal table; it's a current line index. Whereas sy-index in used to find the number of current pass in the loop statement.
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Unique and non-unique indexes. Unique indexes are indexes that help maintain data integrity by ensuring that no two rows of data in a table have identical key values. ... When a unique index is defined for a table, uniqueness is enforced whenever keys are added or changed within the index.
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Using the primary key, you can easily identify and find unique rows in the database table. They allow you to update/delete only specific records by uniquely identifying them. The table's data is sorted based on the primary key. They help you avoid inserting duplicate records in a table.
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Primary key allows each row in a table to be uniquely identified and ensures that no duplicate rows exist and no null values are entered. Unique key constraint is used to prevent the duplication of key values within the rows of a table and allow null values. (In oracle, one null is not equal to another null).
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The main difference between unique and distinct is that UNIQUE is a constraint that is used on the input of data and ensures data integrity. While DISTINCT keyword is used when we want to query our results or in other words, output the data.
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SQL*Loader leaves indexes in an Index Unusable state when the data segment being loaded becomes more up-to-date than the index segments that index it. Any SQL statement that tries to use an index that is in an Index Unusable state returns an error. ... There are duplicate keys in a unique index.
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Unique Key. ... Each table can have more than one Unique Constraint. By default, Unique key is a unique non-clustered index. Unique Constraint can not be related with another table's as a Foreign Key.
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There is two difference between them. 1. The not null constraint is by default added to primary key, it means, primary key attribute cannot accept null values, whereas, the attribute declared as unique can accept null values. It is the major difference between the two.
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['NOT NULL - Ensures that a column cannot have a NULL value.', 'UNIQUE - Ensures that all values in a column are different.', 'PRIMARY KEY - A combination of a NOT NULL and UNIQUE. ... ', 'FOREIGN KEY - Uniquely identifies a row/record in another table.']
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Unique index is for performance. Though unique constraints and unique indexes both help with uniqueness, their objectives differ. A unique constraint is meant to enforce data integrity. A unique constraint might create a unique index implicitly, but it does not rely on or need the index to maintain data integrity.