ALL, DISTINCT, DISTINCTROW, TOP Predicates (Microsoft Access SQL)
|Access Developer Reference|
Specifies records selected with SQL queries.
SELECT [ALL | DISTINCT | DISTINCTROW | [TOP n [PERCENT]]] FROM table
A SELECT statement containing these predicates has the following parts:
|ALL||Assumed if you do not include one of the predicates. The Microsoft Access database engine selects all of the records that meet the conditions in the SQL statement. The following two examples are equivalent and return all records from the Employees table:
|DISTINCT||Omits records that contain duplicate data in the selected fields. To be included in the results of the query, the values for each field listed in the SELECT statement must be unique. For example, several employees listed in an Employees table may have the same last name. If two records contain Smith in the LastName field, the following SQL statement returns only one record that contains Smith:
If you omit DISTINCT, this query returns both Smith records.
If the SELECT clause contains more than one field, the combination of values from all fields must be unique for a given record to be included in the results.
The output of a query that uses DISTINCT is not updatable and does not reflect subsequent changes made by other users.
|DISTINCTROW||Omits data based on entire duplicate records, not just duplicate fields. For example, you could create a query that joins the Customers and Orders tables on the CustomerID field. The Customers table contains no duplicate CustomerID fields, but the Orders table does because each customer can have many orders. The following SQL statement shows how you can use DISTINCTROW to produce a list of companies that have at least one order but without any details about those orders:
If you omit DISTINCTROW, this query produces multiple rows for each company that has more than one order.
DISTINCTROW has an effect only when you select fields from some, but not all, of the tables used in the query. DISTINCTROW is ignored if your query includes only one table, or if you output fields from all tables.
|TOP n [PERCENT]||Returns a certain number of records that fall at the top or the bottom of a range specified by an ORDER BY clause. Suppose you want the names of the top 25 students from the class of 1994:
If you do not include the ORDER BY clause, the query will return an arbitrary set of 25 records from the Students table that satisfy the WHERE clause.
The TOP predicate does not choose between equal values. In the preceding example, if the twenty-fifth and twenty-sixth highest grade point averages are the same, the query will return 26 records.
You can also use the PERCENT reserved word to return a certain percentage of records that fall at the top or the bottom of a range specified by an ORDER BY clause. Suppose that, instead of the top 25 students, you want the bottom 10 percent of the class:
The ASC predicate specifies a return of bottom values. The value that follows TOP must be an unsigned Integer.
TOP does not affect whether or not the query is updatable.
|table||The name of the table from which records are retrieved.|
This example creates a query that joins the Customers and Orders tables on the CustomerID field. The Customers table contains no duplicate CustomerID fields, but the Orders table does because each customer can have many orders. Using DISTINCTROW produces a list of companies that have at least one order but without any details about those orders.