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Chapter 8: Problem 5

Describe conceptually how an SQL query will be executed by specifying the conceptual order of executing each of the six clauses.

Short Answer

Expert verified
The SQL query execution order is: (1) FROM, (2) WHERE, (3) GROUP BY, (4) HAVING, (5) SELECT, (6) ORDER BY.

Step by step solution

01

FROM clause

The execution of an SQL query starts with the FROM clause. This clause is used to specify the tables from which to retrieve data.
02

WHERE clause

After the database knows the table(s) to use via the FROM clause, the WHERE clause is processed. This clause filters the data based on certain conditions, and only rows that meet these conditions are selected.
03

GROUP BY clause

Next, the GROUP BY clause is executed. This clause groups selected rows into set of aggregated rows (groups) based on specified column(s). After this step, we are left with data in aggregated groups for further operations.
04

HAVING clause

The forth clause that is executed is the HAVING clause. Similar to WHERE clause, it filters the data but the key difference is that it works on aggregated data which we get after the execution of GROUP BY clause.
05

SELECT clause

Then, the SELECT clause is processed. Here, specific columns of data within the filtered and grouped rows are chosen. It is at this step that the data to be returned is determined.
06

ORDER BY clause

Finally, the ORDER BY clause is executed. This clause sorts the chosen data in ascending or descending order. The result of the query after this step is the actual output returned by the SQL query.

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Most popular questions from this chapter

Describe the six clauses in the syntax of an SQL query, and show what type of constructs can be specified in each of the six clauses. Which of the six clauses are required and which are optional?

Discuss how NULLs are treated in comparison operators in SQL. How are NULLs treated when aggregate functions are applied in an SQL query? How are NULLs treated if they exist in grouping attributes?

a. Design a relational database schema for your database application. b. Declare your relations, using the SQL DDL. c. Specify a number of queries in SQL that are needed by your database application. d. Based on your expected use of the database, choose some attributes that should have indexes specified on them. e. Implement your database, if you have a DBMS that supports SQL.

List the data types that are allowed for SQL attributes.

How can the key and foreign key constraints be enforced by the DBMS? Is the enforcement technique you suggest difficult to implement? Can the constraint checks be executed efficiently when updates are applied to the database?
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