A Guide to SQL, Seventh Edition

Objectives Retrieve data from a database using SQL commands Use compound conditions Use computed columns Use the SQL LIKE operator Use the SQL IN operator Sort data using the ORDER BY clause A Guide to SQL, Seventh Edition

Objectives Sort data using multiple keys and in ascending and descending order Use SQL aggregate functions Use subqueries Group data using the GROUP BY clause Select individual groups of data using the HAVING clause Retrieve columns with null values A Guide to SQL, Seventh Edition

Constructing Simple Queries Important feature of DBMS is ability to answer a wide variety of questions about the data A query is a question represented in a way that the DBMS can understand Use the SELECT command to query a database A Guide to SQL, Seventh Edition

Constructing Simple Queries SELECT-FROM-WHERE is the basic form of the command SELECT clause is the list of columns to include in query results FROM clause is the name of the table with the data being queried WHERE clause is optional, listing any conditions to apply to the data A Guide to SQL, Seventh Edition

Retrieving Certain Columns and Rows A command can retrieve specified columns and all rows  List the number, name and balance of all customers No WHERE clause is needed, because all customers are requested A Guide to SQL, Seventh Edition

Retrieving All Columns and Rows Use an asterisk (*) to indicate all columns in the SELECT clause Results will list all columns in the order in the description when the table was created List columns in SELECT clause to present columns in a different order A Guide to SQL, Seventh Edition

Using a WHERE Clause WHERE clause is used to retrieve rows that satisfy some condition  What is the name of customer number 148? A simple condition form: column name, comparison operator, and then either a column name or a value A Guide to SQL, Seventh Edition

Using a WHERE Clause Character values in SQL are case sensitive  “Grove” is different than “grove” Simple conditions can compare columns  WHERE BALANCE > CREDIT_LIMIT A Guide to SQL, Seventh Edition

Using Compound Conditions Compound conditions connect two or more simple conditions with AND, OR, and NOT operators  AND operator shows results that all simple conditions are true  OR operator shows results that any simple condition is true  NOT operator reverses the truth of the original condition A Guide to SQL, Seventh Edition

Using the BETWEEN Operator Not an essential feature in SQL Same results can be obtained without it Does make certain SELECT commands simpler to construct BETWEEN operator is inclusive  When using BETWEEN 2000 and 5000, values of 2000 or 5000 would be true A Guide to SQL, Seventh Edition

Using Computed Columns Computed column does not exist in the database but is computed using data in existing columns Computations can involve arithmetic operators  + for addition  - for subtraction  * for multiplication  / for division A Guide to SQL, Seventh Edition

Using the LIKE Operator Used to retrieve data where there may not be an exact match using wildcards LIKE %Central% will retrieve data with those characters  “3829 Central” or “Centralia” Underscore (_) represents any single character  “T_M” for TIM or TOM or T3M A Guide to SQL, Seventh Edition

Using the IN Operator IN operator allows for concise phrasing of certain conditions A Guide to SQL, Seventh Edition

Sorting ORDER BY clause to list data in a specific order Column on which data is to be sorted is the sort key Use ORDER BY clause followed by sort key Rows are sorted in ascending order unless another order is specified A Guide to SQL, Seventh Edition

Additional Sorting Options Possible to sort data by more than one key Major sort key and minor sort key List sort keys in order of importance in the ORDER BY clause For descending order sort, use DESC A Guide to SQL, Seventh Edition

Using Functions Aggregate functions calculate sums, averages, counts, minimum and maximum values to groups of rows A Guide to SQL, Seventh Edition

Using the COUNT Function Counts the number of rows in a table Use of an asterisk allowed to represent any column A Guide to SQL, Seventh Edition

Using the SUM Function Used to calculate totals of columns Column to be summed must be specified and must be numeric AVG, MAX, and MIN functions are similar, resulting in different statistics Null values are ignored and not used in these calculations A Guide to SQL, Seventh Edition

Using the DISTINCT Operator Used to ensure uniqueness in the data results A Guide to SQL, Seventh Edition

Nesting Queries Some queries will take two or more steps to obtain desired results A subquery is an inner query placed inside another query Outer query can use results of the subquery to find its results A Guide to SQL, Seventh Edition

Grouping Grouping creates groups of rows that share common characteristics Calculations in the SELECT command are performed for entire groups Data can be GROUPED BY a particular column, such as REP_NUM and then the statistics are calculated One line of output is produced for each group A Guide to SQL, Seventh Edition

Using a HAVING Clause The HAVING clause is used to restrict groups that will be included A Guide to SQL, Seventh Edition

Nulls Sometimes a condition involves a column that can be null A Guide to SQL, Seventh Edition

Summary Create queries that retrieve data from single tables using SELECT commands Comparison operators: =, >,=>,, or != Compound conditions using AND,OR, and NOT Use the BETWEEN operator Use the LIKE operator A Guide to SQL, Seventh Edition

Summary Use the IN operator Use and ORDER BY clause to sort data Process aggregate functions with COUNT, SUM,AVG,MAX, and MIN Use the DISTINCT operator and subqueries Use GROUP BY, HAVING and IS NULL A Guide to SQL, Seventh Edition