A. The SELECT statement is used to select data from the database. The results are stored in a result table, called a result set.
SELECT
column_name,
column_name
FROM table_name;
FROM table_name;
SELECT * FROM
table_name;
Two. SELECT DISTINCT statement is used to return only difference values. (De-emphasis)
In the table, the column may contain a plurality of duplicate values, different lists (DISTINCT) value.
DISTINCT keyword is used to return only difference values.
SELECT DISTINCT column_name,column_name
FROM table_name;
- Multiple columns to reuse separated. They were applied to each column deduplication
- distinct must be close to the select use: select name, distinct from the Table of the type; - will prompt an error
- Fields distinct statement can only be displayed in select distinct fields specified
-
SELECT COUNT (DISTINCT TYPE) FROM SYS_DICT --DISTINCT no more columns