Execute Immediate and Dynamic SQL

When working with PL/SQL you may find that you would like to iteratively execute a query with different parameters.  Well, got you good news.  You can!

There’s an instruction that will make those ideas come true.

EXECUTE IMMEDIATE is an Oracle implementation that allows the developer to prepare a statement and then execute it from inside the PL/SQL block, defining also input and/or output parameters.

More so, this instruction will generate some tracing info that can be very useful when trying to model the business logic in the pl/sql program.

In its most basic form, it’s used like this:

EXECUTE IMMEDIATE <sql_query>;

This <sql_query> can be as simple or as complex as we want.  It can even be another pl/sql block.  So, you can use a variable to store the statement, let’s say v_smnt and then execute it with:

EXECUTE IMMEDIATE

v_smnt;

Even more, you can use bind variables or parameters with the simple addition of the USING and RETURNING INTO keywords followed by the pl/sql environment variables you want to use.

All these features make this command very powerful.  It also keep some tracking info in memory that you can access through some special variables, like SQL%ROWCOUNT.

Now that you know what would you be looking for, you can visit the next few links for more info on this command and its uses.

Oracle Docs - Command Diagram and elements explanation

Oracle Docs - Dynamic SQL explanation and some examples

And, of course, you can make questions here and I’ll answer as soon as I can.

How to: I need an empty Database – Part 2

In the first part of his post I showed the way to use IMP/EXP utilities from Oracle to get the DDL of a single schema or an entire database.  Some people don’t fully trust these utilities or just dislike the command line and prefer to work over procedures.  This post will give you another option to get a clean database using the information of the data dictionary on any DBMS or the user_<whatever> tables or the all_<whatever> tables from Oracle.
So, here we go.

Oracle

In particular, for the Oracle DBMS, I’ll use a package.  Let’s call it SCHEMA_CLEANER.  Now, we have to state what do we need?  For any database (that respects the relations between tables and data within those tables), the problems when you have to delete some data begin with the foreign keys being violated.
This indicates that the first thing to do is to DISABLE all those constraints so that we can delete all data in the schema without having to take core of the order we’re deleting that data.  But of course, we have later to take the time to enable all those constraints again, because we want to work with a consistent schema, and leaving the constraints disabled will lead almost for sure to inconsistent data.
Following, this we got 2 specifications for our package: DISABLE_FKS and ENABLE_FKS.
Finally, we need the procedure that will do the bulk delete.  So that’s one more specification for the package, let’s say: DELETE_DATA.
For his example I’ll be running the package procedures from the schema we want to clean.  Nevertheless, this code can be easily modified to be run as a DBA from the SYSTEM schema passing the schema to clean up as a parameter.
In this code, then I’ll use the user_<whatever> tables to get the info I need.  The mod would be to use the all_<whatever> tables and filter through the OWNER field in each table.
So, here is the code:

CREATE OR REPLACE PACKAGE SCHEMA_CLEANER AS

PROCEDURE DISABLE_FKS;
PROCEDURE ENABLE_FKS;
PROCEDURE DELETE_DATA;
PROCEDURE CLEAN_SCHEMA;

END SCHEMA_CLEANER;
/


CREATE OR REPLACE PACKAGE BODY SCHEMA_CLEANER AS

PROCEDURE DISABLE_FKS AS
ctable user_tables.table_name%TYPE;
fkey user_constraints.constraint_name%TYPE;

CURSOR tableList IS
SELECT table_name
FROM user_tables;

CURSOR FKList IS
SELECT constraint_name
FROM user_constraints
WHERE table_name = ctable
AND constraint_name LIKE '%FK%';

BEGIN
dbms_output.enable(1000000);
dbms_output.put_line('About to disable foreign keys...');
OPEN tableList;

FETCH tableList INTO ctable;

WHILE tableList%FOUND
LOOP
dbms_output.put_line('Table found: ' || ctable);
OPEN FKList;
FETCH FKList INTO fkey;
WHILE FKList%FOUND
LOOP
EXECUTE IMMEDIATE ('ALTER TABLE ' || ctable || ' DISABLE CONSTRAINT ' || fkey);
dbms_output.put_line('Foreign Key ' || fkey || ' disabled.');
FETCH FKList INTO fkey;
END LOOP;
CLOSE FKList;
FETCH tableList INTO ctable;
END LOOP;    
CLOSE tableList;
dbms_output.put_line('Foreign keys disabled.');
END DISABLE_FKS;

PROCEDURE ENABLE_FKS AS
ctable user_tables.table_name%TYPE;
fkey user_constraints.constraint_name%TYPE;

CURSOR tableList IS
SELECT table_name
FROM user_tables;

CURSOR FKList IS
SELECT constraint_name
FROM user_constraints
WHERE table_name = ctable
AND constraint_name LIKE '%FK%';

BEGIN
dbms_output.enable(1000000);
dbms_output.put_line('About to enable foreign keys...');
OPEN tableList;

FETCH tableList INTO ctable;

WHILE tableList%FOUND
LOOP
dbms_output.put_line('Table found: ' || ctable);
OPEN FKList;
FETCH FKList INTO fkey;
WHILE FKList%FOUND
LOOP
dbms_output.put_line('Foreign Key found: ' || fkey);
EXECUTE IMMEDIATE ('ALTER TABLE ' || ctable || ' ENABLE CONSTRAINT ' || fkey);
dbms_output.put_line('Foreign Key ' || fkey || ' enabled.');
FETCH FKList INTO fkey;
END LOOP;
CLOSE FKList;
FETCH tableList INTO ctable;
END LOOP;    
CLOSE tableList;
dbms_output.put_line('Foreign keys enabled.');
END ENABLE_FKS;

PROCEDURE DELETE_DATA AS
thetable user_tables.table_name%TYPE;

CURSOR ctable IS
SELECT table_name
FROM user_tables;

BEGIN
dbms_output.enable(1000000);
dbms_output.put_line('Starting data deletion process...');
OPEN ctable;
FETCH ctable INTO thetable;
dbms_output.put_line('Deleting, please wait...');
WHILE ctable%FOUND LOOP
dbms_output.put_line('Table found: '|| thetable);
dbms_output.put_line('Deleting data in '|| thetable);
EXECUTE IMMEDIATE ('DELETE FROM ' || thetable);
dbms_output.put_line(thetable || ' is empty.');
FETCH ctable INTO thetable;
END LOOP;
CLOSE ctable;
dbms_output.put_line('Data deletion process finished.');
END DELETE_DATA;

PROCEDURE CLEAN_SCHEMA AS
BEGIN
DBMS_OUTPUT.ENABLE(1000000);
SCHEMA_CLEANER.DISABLE_FKS;
SCHEMA_CLEANER.DROP_DYNAMIC_TABLES;
SCHEMA_CLEANER.DELETE_DATA;
SCHEMA_CLEANER.ENABLE_FKS;
END CLEAN_DB;

END SCHEMA_CLEANER;

/

How to: I need an empty Database! – Part 1

In your daily work, for various reasons you may need an empty database.  May be for testing purposes, for testing bulk data load or to identify changes over tables caused by an application that is working over the database.

Whichever the case may be, you need a clean database and not always got the time (or the option) to start following relational constraints backwards.

Here’s an option to get an empty database out from a fully working online one.

Import / Export  Utilities (Oracle only)

For the task at hand, using the IMP/EXP utilities can be quite helpful and fast. Both of them provide the option to import/export the full database, only a schema and with or without data.

If you don’t have any .DMP file or just want to create a new one, you can use the following command to export the database DDL.

EXP SYSTEM/mypswd FILE=fulldb.dmp FULL=Y ROWS=N

Or as well you can make a parameters file containing this information

FILE=fulldb.dmp
FULL=Y
ROWS=N
LOG=export.log

and then use it as a parameter itself as follows

EXP SYSTEM/mypswd PARFILE=myparams.dat

Now, if you do have a DMP generated, let’s say, by a backup policy, then you can just use the IMP utility to import the DDL mostly the same way as before with just some little changes.

Let’s state that when someone says “I need an empty database” it really means (usually) “I need an empty schema”

So, you must specify (to avoid loosing data dictionary and other important data) the parameters FROMUSER and TOUSER.  This will tell the utility which schema we want to import, as follows

IMP SYSTEM/mypswd FILE=fulldb.dmp FROMUSER=my_schema TOUSER=another_schema

In the next part, I’ll post some procedures that can be used to clean any schema of the database.  For Oracle, they can be packaged, for other DBMS you just can run the stored procedures.

As always, hope this helps…  And comments are welcome!

Good Practices for Creation Scripts - Part 1

When you're working with small databases with just a few tables, everything is handled very fast and easy.

But what happens when our small application starts to grow and some modules appear and require more and more tables? Why not start doing things the right way to avoid future "complete reconstructions" due to incompatibilities?

So, here are some tips:

• For each table, always have a code (2 to 4 chars)

employee --> emp
department --> dep
title --> ttl
works_in --> wrk

• Each column in the table must contain the code of its parent table. Foreign keys will have the referenced table code.

In table employee:
id --> emp_id
name --> emp_name
title --> ttl_id

• Always write complete creation scripts. Avoid using simplified coding style, because it will generate constraints with system-generated names, hard to find and identify later on. When a constraint is violated, it’s good to have it named to avoid encounters with messages like “Constraint SYS00013292384 violated”.
  

I'll present 2 ways to create a very small employee database, make your own conclusions:

The short way:

CREATE TABLE title (
id NUMBER(10) PRIMARY KEY,
desc VARCHAR2(50) NOT NULL
);

CREATE TABLE employee (
id NUMBER(12) PRIMARY KEY,
name VARCHAR2(50) NOT NULL,
title NUMBER(10) NOT NULL
);
ALTER TABLE employee ADD FOREIGN KEY (title) REFERENCES title (id);

CREATE TABLE department (
id NUMBER(2) PRIMARY KEY,
name VARCHAR2(50) NOT NULL
);

CREATE TABLE works_in (
emp NUMBER(12) NOT NULL,
dep NUMBER(2) NOT NULL
);
ALTER TABLE works_in ADD PRIMARY KEY (emp,dep);
ALTER TABLE works_in ADD FOREIGN KEY (emp) REFERENCES employee (id);
ALTER TABLE works_in ADD FOREIGN KEY (dep) REFERENCES department (id);

The right way (for me) to save us trouble in the future (and start making good coding habits):

CREATE TABLE title
(
ttl_id NUMBER(2),
ttl_name VARCHAR(50)
);

ALTER TABLE title ADD CONSTRAINT "PK_ttl" PRIMARY KEY (ttl_id);


CREATE TABLE employee
(
emp_id NUMBER(12),
emp_name VARCHAR(50),
ttl_id NUMBER(2),
);

ALTER TABLE employee
ADD CONSTRAINT "PK_emp" PRIMARY KEY (emp_id);
ALTER TABLE employee
ADD CONSTRAINT "NN_emp_name" CHECK (emp_name IS NOT NULL);
ALTER TABLE employee
ADD CONSTRAINT "FK_emp_ttl" FOREIGN KEY (ttl_id)
REFERENCES title (ttl_id);


CREATE TABLE department
(
dep_id NUMBER(2),
dep_name VARCHAR(25),
dep_location VARCHAR(50)
);

ALTER TABLE department
ADD CONSTRAINT "PK_dep" PRIMARY KEY (dep_id);

CREATE TABLE works_in
(
emp_id NUMBER(12),
dep_id NUMBER(2)
);

ALTER TABLE works_in
ADD CONSTRAINT "PK_wrk" PRIMARY KEY (emp_id, dep_id);

ALTER TABLE works_in
ADD CONSTRAINT "FK_wrk_emp" FOREIGN KEY (emp_id)
REFERENCES employee (emp_id);

ALTER TABLE works_in
ADD CONSTRAINT "FK_wrk_dep" FOREIGN KEY (dep_id)
REFERENCES department (dep_id);

If you use these simple tips, you’ll not only work in a more organized way, you’ll be more efficient when solving problems over databases with these kind of implementation.

Once again, I’m not telling this is THE WAY to do things.  Just one way that works for me.  Comments are always welcome.