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SELECT * FROM (SELECT ROW_NUMBER OVER (ORDER BY sort_key ASC) AS row_number, columns FROM tablename) AS foo WHERE row_number <= 10 ROW_NUMBER can be non-deterministic : if sort_key is not unique, each time you run the query it is possible to get different row numbers assigned to any rows where sort_key is the same.
Small number of rows The cost of full table scan is less than index range scan due to small table. When query processed SELECT COUNT(*), nulls existed in the column The query is counting the number of null columns in a typical index. However, SELECT COUNT(*) can't count the number of null columns. The query is unselective The number of return ...
In databases, a partial index, also known as filtered index is an index which has some condition applied to it so that it includes a subset of rows in the table.. This allows the index to remain small, even though the table may be rather large, and have extreme selectivity.
A common table expression, or CTE, (in SQL) is a temporary named result set, derived from a simple query and defined within the execution scope of a SELECT, INSERT, UPDATE, or DELETE statement. CTEs can be thought of as alternatives to derived tables ( subquery ), views , and inline user-defined functions.
In relational databases, the information schema (information_schema) is an ANSI-standard set of read-only views that provide information about all of the tables, views, columns, and procedures in a database. [1] It can be used as a source of the information that some databases make available through non-standard commands, such as:
An index is a copy of selected columns of data, from a table, that is designed to enable very efficient search. An index normally includes a "key" or direct link to the original row of data from which it was copied, to allow the complete row to be retrieved efficiently. Some databases extend the power of indexing by letting developers create ...
This table contains the following columns: Column1, Column2, ..., Columnn. The column Column1 is assumed to be the primary key. These columns are defined to have the following types: Type1, Type2, ..., Typen. The Log Trigger works writing the changes (INSERT, UPDATE and DELETE operations) on the table in another, history table, defined as ...
For example, in Microsoft SQL Server, the key is retrieved via the SCOPE_IDENTITY() special function, while in SQLite the function is named last_insert_rowid(). Using a database-specific SELECT statement on a temporary table containing last inserted row(s). Db2 implements this feature in the following way: