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The set of values in a column is defined in a column of another table that contains unique values. For example, in a US taxpayer database, the "state" column is required to belong to one of the US's defined states or territories: the set of permissible states and territories is recorded in a separate table.
This is especially true of cryptographic hash functions, which may be used to detect many data corruption errors and verify overall data integrity; if the computed checksum for the current data input matches the stored value of a previously computed checksum, there is a very high probability the data has not been accidentally altered or corrupted.
QID (or Q number) is the unique identifier of a data item on Wikidata, comprising the letter "Q" followed by one or more digits. It is used to help people and machines understand the difference between items with the same or similar names, e.g., there are several places in the world called London and many people called James Smith.
In the figure, the fraction 1/9000 is displayed in Excel. Although this number has a decimal representation that is an infinite string of ones, Excel displays only the leading 15 figures. In the second line, the number one is added to the fraction, and again Excel displays only 15 figures. In the third line, one is subtracted from the sum using ...
For example, this could be used to find duplicate billings of invoices within the sales ledger. Matching (Fuzzy): Specifies whether the product supports finding matching items for a specific field using fuzzy comparison. For instance, values compared are similar but not exactly the same (e.g., using Levenshtein matching).
In the phone book example with a composite index created on the columns (city, last_name, first_name), if we search by giving exact values for all the three fields, search time is minimal—but if we provide the values for city and first_name only, the search uses only the city field to retrieve all matched records.
The Spreadsheet Value Rule. Computer scientist Alan Kay used the term value rule to summarize a spreadsheet's operation: a cell's value relies solely on the formula the user has typed into the cell. [48] The formula may rely on the value of other cells, but those cells are likewise restricted to user-entered data or formulas.
For referential integrity to hold in a relational database, any column in a base table that is declared a foreign key can only contain either null values or values from a parent table's primary key or a candidate key. [2] In other words, when a foreign key value is used it must reference a valid, existing primary key in the parent table.