Basic Mysql

Basic-MySQL

The basic query has three clauses: SELECT, FROM, and WHERE.

  • SELECT: contains one or multiple attributes. These attributes are displayed in the result. The symbol “*” means all attributes.
  • The FROM clause contains one or more tables.
  • The WHERE clause contains a single predicate. It is a logical test on every row of the table which returns true or false. If multiple queries are executed at the same time, a semicolon “;” is used as a delimiter to split two queries.

  • A query is executed as follows. The system test the predicate on every tuple from the table in the FROM clause. If a tuple satisfies the predicate, show the values of the attributes in the SELECT clause in the result.

Predicates

  1. The predicate in the WEHERE clause is regarded as a logic test. The return value of a predicate is a Boolean, either 1 (True) or 0 (False).
  2. Formally, suppose op is a relation operator, which can be =, >, >=, <, <=, and <> (not equal); a predicate can be a single term
  pred = term
  term = exp op exp

“exp” is an arithmetic expression which contains attributes and constants.

For example

rental_rate < 1
  1. A predicate can also be a composition of terms.
  pred = NOT pred
  pred = pred AND pred
  pred = pred OR pred

NOT, AND, OR are logical operators. For example,

NOT rental_rate < 1
(NOT rental_rate < 1) AND release_year = 2006
SELECT * FROM film WHERE rating="PG-13"

SQL is case insensitive. However, to make query readable, we write key words (like SELECT, FROM, etc.) in capital; attributes and tables in lower cases; and string constant in the original form.

In general, a basic query is in the form

SELECT a1,,an FROM r WHERE P

where $a_1,⋯,a_n$ are attributes; r is a table; and P is a predicate.

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