CpSc 3220 The Language of SQL Chapters 7-9

The WHERE Clause Determines which rows of table are to be selected

Syntax of the SELECT statement SELECT [ALL | DISTINCT | DISTINCTROW ] [HIGH_PRIORITY] [STRAIGHT_JOIN] [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT] [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS] select_expr [, select_expr...] [FROM table_references [WHERE where_condition] [GROUP BY {col_name | expr | position} [ASC | DESC],... [WITH ROLLUP]] [HAVING where_condition] [ORDER BY {col_name | expr | position} [ASC | DESC],...] [LIMIT {[offset,] row_count | row_count OFFSET offset}] [PROCEDURE procedure_name(argument_list)] [INTO OUTFILE 'file_name' [CHARACTER SET charset_name] export_options | INTO DUMPFILE 'file_name' | INTO var_name [, var_name]] [FOR UPDATE | LOCK IN SHARE MODE]]

Row-based Logic The where_condition term in the WHERE clause is applied to each row and only those for which the condition is true are selected The where_condition is very similar to conditional expressions in other languages such as Java and PHP It can be comprised of column variables, literals and operators

WHERE Clause Comparison Operators =equals <>does not equal !=does not equal >is greater than <is less than >=is greater than or equal to <=is less than or equal to

Limiting Rows In MySQL the LIMIT clause can be used to return only a specified number of rows LIMIT n returns the ‘first’ n rows If used after an ORDER clause it will return the first n rows of the sorted table Other DBMSs handle this process differently

Boolean Logic Boolean operators can be combined with conditional operators to express more complex logical conditions

The AND Operator Returns true if both operands are true WHERE age > 21 AND class = ‘FRESHMAN’

The OR Operator Returns true if either or both operands are true Returns false otherwise WHERE age > 21 OR class = ‘FRESHMAN’

The XOR Operator Returns true if one but not both operands are true Returns false otherwise WHERE age > 21 XOR class = ‘FRESHMAN’ Available in MySQL but not all other DBMSs

The NOT Operator A unary operator Returns true if operand is false Returns false if operand is ture

Precedence AND takes precedence over OR and XOR NOT takes precedence over AND All boolean operators take precedence over conditional operators Parentheses can be used to override precedence

The BETWEEN Operator Example: WHERE hours BETWEEN 30 AND 59

The IN Operator Examples WHERE age IN (20,24,45) WHERE color IN (‘Red’,’White’,’Blue’)

Boolean Logic and NULL Values The conditional operators do not work on NULL values The ISNULL function can be used ISNULL(operand,result) returns result if operand is NULL, returns operand otherwise Example: ISNULL(Weight,0)

Inexact Matches SQL has several ways of handling inexact matches – LIKE – REGEXP (supported by MySQL) – SOUNDEX – DIFFERENCE (not supported by MySQL)

Pattern Matches Using LIKE WHERE operand LIKE ‘pattern’ pattern has two special symbols, % and _ % matches any string of any length _ matches a single character

Example Matches Using LIKE WHERE name LIKE ‘%ie’ finds all names ending in ie WHERE name LIKE ‘TH%’ finds all names starting with th WHERE name LIKE ‘%ing%’ finds all name containing ing Matches are case-insensitive

Pattern Matches Using REGEXP MySQL supports a REGEXP operator that functions much like the REGEXP in PHP WHERE name REGEXP regexpPattern uses the regexpPattern to match the name operand REGEXP is much the same as the regular expression function in PHP

Matches by Sound There is a SOUNDEX function which converts a string into a code that reflects its ‘sound value’ and this can be used to find two strings that have the same sound code.