Oracle NVL Equivalent in MySQL
Here's the MySQL equivalent of Oracle's NVL and MSSQL/Sybase's ISNULL functions.
Syntax
IFNULL(expr1,expr2)
Usage
If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2. Take note that IFNULL() returns a numeric or string value, depending on the context in which it is used. I haven't used it for other data types yet but I reckon it would have the same results. That is to return the same data type as that of expr1. Another important thing to remember is that expr1 and expr2 should have the same data type
MySQL SEQUENCE Equivalent
Most Oracle guys (and I heard PostgreSQL, too) will be missing sequence in MySQL. The nearest equivalent that MySQL has for this feature is the AUTO INCREMENT field. However, auto increment is sometimes not sufficient and time will come that you will be missing it.
Case in point is the alpha-numeric auto-generated code. In Oracle, it's easy to generate AA123, AA124, AA125, etc., etc. All we have to do is concat the alpha part (AA) and with the nextval of sequence. For MySQL, the alpha part and the numeric part have to be saved in separate fields.
However, Maresa of Microshell has created an alternative solution for it. The article discusses a way to emulate the nextval function in mysql. It a nutshell, they created a table similar to the all_sequences of Oracle. This is where you insert your sequences. Then, create a function named nextval() function which increments and returns value of the sequence.
You can check the procedure here.
It's pretty neat, actually. It's an excellent alternative and you can even create similar functions like currval(), prevval(), among others. However, I have some reservations on his approach especially on the issue of concurrency. You see in Oracle, calling the nextval() will increment the value and that's already persisted (saved) in the sequence table even without saving your query/dml statement. On this approach, other queries accessing the pseudo-nextval will have to wait until your query finishes. Not a good idea.
So for now, I'll will still have to make do with whatever MySQL is providing. If there's any consolation, MySQL provides LAST_INSERT_ID() function in case you want to get the latest id inserted. See details here here.
So I'm just hoping that MySQL will implement an equivalent feature in the near future. With the imminent takeover by Oracle, I'm actually hoping that development of MySQL will push through and take a steam. We all know that the development of this widely used open-source database engine has been hampered ever since Sun bought it.
Sequence is a little feature. But little as it seems, it solves a lot of problems and helps improve code development time.
Oracle WITH Clause (Subquery Factoring)
WITH clause was introduced in Oracle 9i to provide powerful new syntax for enhancing query performance. It optimizes query speed by eliminating redundant processing in complex queries.
WITH Clause Syntax
WITH alias_name -- alias for the aggregate_query AS (aggregate_query_here) SELECT... -- Beginning of the query main body
It should be noted that multiple aggregate queries can be defined in the WITH clause as below.
WITH alias_name1 AS (agg_query1) , alias_name2 AS (agg_query2) SELECT...
When using subquery factoring, think of the aggregate_query as an in-line view. Actually, it is a view but a temporary one. Instead of creating a permanent view accessible in the database, you are creating a temporary one exclusively used by your main query.
Usage
I primarily use this in creating adhoc queries, reports and extractions. Of course, you can just create a view to accomplish this task. However, there are times wherein you are not allowed to create additional objects in the database for one reason or another. In that case, this subquery optimization comes very handy.
Caveat
You can't use this inside Oracle Forms. It also doesn't work inside stored procedures or anything with PL/SQL in it. At least I'm talking about until 11g.
Oracle CASE WHEN Statement
Another oracle sql statement that you may need in order to simplify and optimize your code is the use of CASE statement. Here's how it is being used.
Syntax #1
CASE [ expression ]
WHEN condition_1 THEN result_1
WHEN condition_2 THEN result_2
...
WHEN condition_n THEN result_n
ELSE result
END
Example
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;
Syntax #2
CASE
WHEN [ condition_1 ] THEN result_1
WHEN [ condition_2 ] THEN result_2
...
WHEN [ condition_n ] THEN result_n
ELSE result
END
Example
SELECT m.paper_type ,
m.paper_brand ,
m.paper_grammage,
m.paper_width ,
m.paper_length ,
m.paper_stock_no,
(
CASE
WHEN NVL(smc.qty_reserved_onhand,0) != NVL(st.sum_qty_r_oh,0) THEN 'QROH NOT TALLY'
WHEN NVL(smc.qty_reserved_incom,0) != NVL(st.sum_qty_r_incom,0) THEN 'QRINC NOT TALLY'
WHEN NVL(smc.ton_reserved_onhand,0) != NVL(st.sum_ton_r_oh,0) THEN 'TROH NOT TALLY'
WHEN NVL(smc.ton_reserved_incom,0) != NVL(st.sum_ton_r_incom,0) THEN 'TRINC NOT TALLY'
END
) ERROR_MESSAGE
from paper_stock_master m
Personally, I use Syntax #2 most of the time. I don't know why but maybe it just happened that I find it more suitable to use in the situations that I needed the CASE statement.
Difference Between DECODE and CASE WHEN
The most fundamental difference between the two oracle statements is that in DECODE, you are comparing a field or value in a given set of values or fields. It's actually a shorthand for IF..ELSIF..ELSE statement with the condition all set to equals (=). CASE WHEN is almost the same but you can use any conditional operator (i.e, =,!=, >, >= , <, <=, etc) thereby giving you more freedom and flexibility in writing your code.
In may instances CASE WHEN helped me reduce the number of lines of my codes as well as optimize the performance. On top of that, I no longer need a full pl/sql program with multiple queries and even cursors to be able to provide the result that a simple CASE statement can provide. However CASE, like any other sql commands, is only applicable for certain scenarios. It's not a magic keyword that will solve/apply to everything. So you have to weigh your options if you need it or not.
Ciao!
Rename Table in Oracle
SYNTAX
alter table
table_name
rename to
new_table_name;
Example
alter table
employee
rename to
employee_bak;
