Advanced Sql Training

Editor's Notes

• #19: Select NULL + 1 FROM dual vs Select NVL(NULL,0) + 1 FROM dual
• #21: SELECT … FROM … WHERE … LIKE (%...) SELECT … FROM … WHERE IN (… ,… ,…) OR SELECT … FROM … WHERE EXISTS (SELECT … FROM … WHERE …) SELECT … FROM … WHERE IN (SELECT … FROM … WHERE …)
• #22: SELECT … FROM … WHERE … LIKE (%...) SELECT … FROM … WHERE IN (… ,… ,…) OR SELECT … FROM … WHERE EXISTS (SELECT … FROM … WHERE …) SELECT … FROM … WHERE IN (SELECT … FROM … WHERE …)
• #25: Keywords ALL and DISTINCT can be used depending on the language to modify set operators to return either the entire result set (ie: duplicates) by specifying ALL or or to exclude the duplicates by specifying DISTINCT. The SQL standard does not enforce keyword Distinct and some DBMSs for example Oracle and SQL Server do not even allow it, therefore if you see just Union, Except or Intersect - these actually mean Union Distinct, Except Distinct and Intersect Distinct. Facts to remember: Column count must be the same; Data types of retrieved columns should match or at least should be implicitly convertible by database; Usually returned column names are taken from the first query; Order by clauses for each individual query except the last one cannot be there at all as is the case with Oracle or are ignored in MySQL.
• #26: Facts to remember: Check DB documentation for the order of operators, because for example Oracle executes operators starting from left to right, but DB2 executes Intersect operators first;
• #27: SELECT … FROM … WHERE … LIKE (%...) SELECT … FROM … WHERE IN (… ,… ,…) OR SELECT … FROM … WHERE EXISTS (SELECT … FROM … WHERE …) SELECT … FROM … WHERE IN (SELECT … FROM … WHERE …)
• #30: SELECT last_name, commission_pct,    (CASE commission_pct      WHEN 0.1 THEN ‘Low’      WHEN 0.15 THEN ‘Average’      WHEN 0.2 THEN ‘High’      ELSE ‘N/A’   END ) Commission FROM employees ORDER BY last_name;   SELECT last_name, job_id, salary, (CASE      WHEN job_id LIKE 'SA_MAN' AND salary < 12000 THEN '10%'   WHEN job_id LIKE 'SA_MAN' AND salary >= 12000 THEN '15%'      WHEN job_id LIKE 'IT_PROG' AND salary < 9000 THEN '8%'      WHEN job_id LIKE 'IT_PROG' AND salary >= 9000 THEN '12%'      ELSE 'NOT APPLICABLE'    END ) Raise FROM employees;  
• #41: Facts to remember Usually cross joins are used quite rarely; some of the scenarios could be as follows: Possibility to generate high amount of rows. As we can see from relatively small tables there is possibility to get quite monstrous numbers. Find out all possible row combinations of some tables. Mostly this is useful for reports where one needs to generate all combinations for example all nationalities x genders for persons. To join a table with just one row. Most often used to get some configuration parameters.
• #42: Facts to remember: All following facts are relevant to almost every join type, however the inner join is the most common join type. Databases are designed to do joins efficiently. Not client, not middle tier, but databases. Creating data model usually normalization is being done mostly to avoid data duplication. To show data in person-readable form, joining is one of the necessary prerequisites. Inner join is one of the most common join types and it should be done in database. Database means efficiently in the place where data resides and not in client/middle tier cycling through (possibly) many cycles. It should be noted that other clauses for SQL statements e.g. GROUP BY, HAVING, ORDER BY are of course usable for SQL statements with joins. One must be cautious using join conditions with columns without NOT NULL constraint. Comparing NULL values with different explicit values or even Nulls is probably a bit counter-intuitive on the first sight. The position of additional conditions is irrelevant for INNER joins. These can be written both as join condition and in WHERE clause, result is the same. For Inner join it is not relevant, which table is first one and which second one.
• #43: Facts to remember: Outer joins should be used only when necessary. If it is possible (data model and business data allows) inner join should be used. Inner join offers greater flexibility for optimizer and doesn't mislead people to the thoughts that some rows of one cannot be joined to other table. Unlike Inner joins it is important where the condition is written - either as join condition or in WHERE clause. Not all Database management systems support Full outer joins. Use a (the set operator) UNION of Left and Right joins to simulate a FULL outer join. Oracle (MySQL) outer join operator has more restrictions than more modern ANSI syntax, for example: It doesn't support Full Outer join; It cannot be used together with modern syntax; Join condition cannot contain logical OR and IN operators.
• #52: If the database system can determine the reverse mapping from the view schema to the schema of the underlying base tables, then the view is updatable. Insert, update and delete operations can be performed on updatable views. Notes on Updatable Views An updatable view is one you can use to insert, update, or delete base table rows. You can create a view to be inherently updatable, or you can create an INSTEAD OF trigger on any view to make it updatable. To learn whether and in what ways the columns of an inherently updatable view can be modified, query the USER_UPDATABLE_COLUMNS data dictionary view. The information displayed by this view is meaningful only for inherently updatable views. For a view to be inherently updatable, the following conditions must be met: Each column in the view must map to a column of a single table. For example, if a view column maps to the output of a TABLE clause (an unnested collection), then the view is not inherently updatable. The view must not contain any of the following constructs: A set operator a DISTINCT operator An aggregate or analytic function A GROUP BY, ORDER BY, MODEL, CONNECT BY, or START WITH clause A collection expression in a SELECT list A subquery in a SELECT list A subquery designated WITH READ ONLY Joins, with some exceptions, as documented in Oracle Database Administrator's Guide In addition, if an inherently updatable view contains pseudocolumns or expressions, then you cannot update base table rows with an UPDATE statement that refers to any of these pseudocolumns or expressions.
• #53: If you want a join view to be updatable, then all of the following conditions must be true: 1.  The DML statement must affect only one table underlying the join. 2.  For an INSERT statement, the view must not be created WITH CHECK OPTION, and all columns into which values are inserted must come from a key-preserved table. A key-preserved table is one for which every primary key or unique key value in the base table is also unique in the join view. 3.  For an UPDATE statement, all columns updated must be extracted from a key-preserved table. If the view was created WITH CHECK OPTION, then join columns and columns taken from tables that are referenced more than once in the view must be shielded from UPDATE. For a DELETE statement, if the join results in more than one key-preserved table, then Oracle Database deletes from the first table named in the FROM clause, whether or not the view was created WITH CHECK OPTION.