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Since at the application level, the majority of queries we code are multi-tables like the one used above, the clumsiness of using a table name even abbreviated as part of column names becomes readily apparent. Of course, the same exception we saw in the Naming Consistency section applies: a column foreign key referring to a lookup table normally includes this table’s name as part of the column’s name. For example, in the car_event table, we have event_code which refers to the code column in table event. Summary To get a clear and understandable data structure, proper data elements naming is important. We examined many techniques to apply in order to build consistent table and column names.
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Naming Consistency We should ensure that a data element that is present in more than one table is represented everywhere by the same column name. In MySQL, a column name does not exist by itself; it is always inside a table. This is why, unfortunately, we cannot pick up consistent column names from, say, a pool of standardized column names and associate it with the tables. Instead, during each table’s creation we indicate the exact column names we want and their attributes. So, let’s avoid using different names internal_number and internal_num when they refer to the same reality. An exception for this: if the column’s name refers to a key in another table the state column and we have more than one column referring to it like state_of_birth, `state_of_residence`. MySQL’s Possibilities versus Portability MySQL permits the use of many more characters for identifiers database, table, and column names than its competitors. The blank space is accepted as are accented characters. The simple trade-off is that we need to enclose such special names with back quotes like ’state of residence’. This procures a great liberty in the expression of data elements, especially for non-English designers, but introduces a state of non-portability because those identifiers are not accepted in standard SQL. Even some SQL implementations only accept uppercase characters for identifiers. I recommend being very prudent before deciding to include such characters. Even when staying faithful to MySQL, there has been a portability issue between versions earlier than 4.1 when upgrading to 4.1. In 4.1.x, MySQL started to represent identifiers internally in UTF-8 code, so a renaming operation had to be done to ensure that no accented characters in the database, table, column and constraint names were present before the upgrade. This tedious operation is not very practical in a 24/7 system availability context. Table Name into a Column Name Another style I often see: one would systematically add the table name as a prefix to ……………………….every………………….. column name. Thus the car table would be comprised of the columns: car_id_number, car_serial_number. I think this is redundant and it shows its inelegance when examining the queries we build: select car_id_number from car is not too bad, but when joining tables we get a query such as select car.car_id_number, buyer.buyer_name from car, buyer
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Clarity versus Length: an Art A consistent style of abbreviations should be used. In general, only the most meaningful words of a sentence should be put into a name, dropping prepositions, and other small words. As an example, let’s take the postal code. We could express this element with different column names: the_postal_code pstl_code pstlcd postal_code I recommend the last one for its simplicity. Suffixing Carefully chosen suffixes can add clarity to column names. As an example, for the date of first payment element, I would suggest first_payment_date. In fact, the last word of a column name is often used to describe the type of content like customer_no, color_code, interest_amount. The Plural Form Another point of controversy for table names: should we use the plural form cars table? It can be argued that the answer is yes because this table contains many cars in other words, it is a set. Nonetheless, I tend not to use the plural form for the simple reason that it adds nothing in terms of information. I know that a table is a set, so using the plural form would be redundant. It can be said also that each row describes one car. If we consider the subject on the angle of queries, we can draw different conclusions depending on the query. A query referring to the car table select car.color_code from car where car.id = 34 is more elegant if the plural form is not used, because the main idea here is that we retrieve one car whose id equals 34. Some other queries might make more sense with a plural, like select count(*) from cars. As a conclusion for this section, the debate is not over, but the most important point is to choose a form ……………………………………….and be consistent throughout the whole system.
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The free_value field itself must be defined with a generic field type like VARCHAR whose size must be wide enough to accommodate all values for all possible corresponding free_name values. It prevents easy validation (for a weight, we need a numeric value). Coding the SQL queries on these free fields becomes more complex i.e. SELECT internal_number from car_free_field where free_name = ‘weight’ and free_value > 2000. Naming Recommendations Here we touch a subject that can become sensitive. Establishing a naming convention is not easily done, because it can interfere with the psychology of the designers. Designer’s Creativity Programmers and designers usually think of themselves as imaginative, creative people; UI design and data model are the areas in which they want to express those qualities. Since naming is writing, they want to put a personal stamp to the column and table names. This is why working as a team for data structure design necessitates a good dose of humility and achieves good results only if everyone is a good team player. Also, when looking at the work of others in this area, there is a great temptation to improve the data elements names. Some discipline in the standardization has to be applied and all the team members have to collaborate. Abbreviations Probably because older database systems had severe restrictions about the representation of variables and data elements in general, the practice of abbreviating has been taught for many years and is followed by many data structure designers and programmers. I used programming languages that accepted only two characters for variable names we had to extensively comment the correspondence between those cropped variables and their meaning. Nowadays, I see no valid reasons for systematically abbreviating all column and table names; after all, who will understand the meaning of your T1 table or your B7 field?
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In the following example, we use the car_free_field table to store unplanned information about the car whose internal_number is 412. The weight and special paint had not been planned, so the UI gave the user the chance to specify which information they want to keep, and the corresponding value. We see here a screenshot from phpMyAdmin but most probably, another UI would be presented to the user for example the salesperson who might not be trained to play at the database level. CREATE TABLE `car_free_field` ( `internal_number` int(11) NOT NULL, `free_name` varchar(30) NOT NULL, `free_value` varchar(30) NOT NULL, PRIMARY KEY (’internal_number’,'free_name’) ) ENGINE=MyISAM DEFAULT CHARSET=latin1; INSERT INTO `car_free_field` VALUES (412, ‘weight’, ‘2000′); INSERT INTO `car_free_field` VALUES (412, ’special paint needed’, ‘gold’); Pitfalls of the Free Fields Technique Even if it’s tempting to use this kind of table for added flexibility and to avoid user interface maintenance, there are a number of reasons why we should avoid using it. It becomes impossible to link this “column” (for example the special paint needed) to a lookup table containing the possible colors, with a foreign key constraint.
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Again, sample values are entered via phpMyAdmin: Data can also hide in a table name. Let’s consider the car and truck tables. They should probably be merged into a vehicle table, since the vehicle’s category truck, car, and other values like minivan is really an attribute of a particular vehicle. We could also find another case for this table name problem: a table named vehicle_1996. Planning for Changes When designing a data structure, we have to think about how to manage its growth and the possible implications of the chosen technique. Let’s say an unplanned car characteristic the weight has to be supported. The normal way of solving this is to find the proper table and add a column. Indeed, this is the best solution; however, someone has to alter the table’s structure, and probably the UI too. The free fields technique, also called second-level data or EAV (Entity-Attribute- Value) technique is sometimes……………………………………………. used in this case. To summarize this technique, we use a column whose value is a column name by itself. Even if this technique is shown here, I do not recommend using it, for the reasons explained in the Pitfalls of the Free Fields Technique section below. The difference between this technique and our car_event table is that, for car_event, the various attributes can all be related to a common subject, which is the event. On the contrary, free fields can store any kind of dissimilar data. This might also be a way to store data specific to a single instance or row of a table.
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Here are the structure and sample values for those tables: CREATE TABLE `event` ( `code` int(11) NOT NULL, `description` char(40) NOT NULL, PRIMARY KEY (’code’) ) ENGINE=MyISAM DEFAULT CHARSET=latin1; INSERT INTO `event` VALUES (1, ‘washed’); The usage of backticks here (’event’), although not standard SQL, is a MySQL extension used to enclose and protect identifiers. In this specific case, it could help us with MySQL 5.1 in which the event keyword is scheduled to become part of the language for some another purpose (CREATE EVENT). At the time of writing, beta version MySQL 5.1.11 accepts CREATE TABLE event, but it might not always be true. The following image shows sample values entered into the event table from within the Insert sub-page of phpMyAdmin: CREATE TABLE `car_event` ( `internal_number` int(11) NOT NULL, `moment` datetime NOT NULL, `event_code` int(11) NOT NULL, PRIMARY KEY (’internal_number’) ) ENGINE=MyISAM DEFAULT CHARSET=latin1; INSERT INTO `car_event` VALUES (412, ‘2006-05-20 09:58:38′, 1);
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By removing the total price column, we could encounter a pitfall. We have to be sure that we can reconstruct this total price from other sub-total elements, now and in the future. This might not be possible for a number of reasons: The total price includes an amount located in another table, and this table will change over time (for example, the tax rate). To avoid this problem, see the recommendations in the Scalability over Time section in Chapter 4. This total price contains an arbitrary value, due to some exceptional cases, for example, where there is a special sale, and the rebate was not planned in the system, or when the lucky buyer is the brother-in-law of the general manager! In this case, a decision can be made: adding a new column other_rebate. Data as a Column’s or Table’s Name Now is the time to uncover what is perhaps the least known of the data naming problems: data hidden in a column’s or even a table’s name. We had one example of this in Chapter 1. Remember the qty_2006_1 column name. Although this is a commonly seen mistake, it’s a mistake nonetheless. We clearly have two ideas here, the quantity and the date. Of course, to be able to use just two columns, some work will have to be done regarding the keys this is covered in Chapter 4. For now, we should just use elements like quantity and date in our elements list, avoiding representing data in a column’s name. To find those problematic cases in our model, a possible method is to look for numbers. Column names like address1, address2 or phone1, phone2 should look suspicious. Now, have a look in Chapter 2 at the data elements we got from our store assistant. Can you find a case of data being hidden in a column name? If you have done this exercise, you might have found many past participles hidden into the column names, like ordered, arrived, and washed. These describe the events that happen to a car. We could try to anticipate all possible events but it might prove impossible. Who knows when a new column car_provided_with_big_ribbon will be needed? Such events, if treated as distinct column names, must be addressed by A change in the data structure A change in the code (UI and reports) To stay flexible and avoid the wide-table syndrome, we need two tables: car_event and event.
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Chapter 3 [ 29 ] As a result, we determine that we should sub-divide the name into the following elements: Salutation First name Last name Sometimes it’s useful to sub-divide an element, sometimes it’s not. Let’s consider the date elements. We could sub-divide each one into year, month, and day (three integers) but by doing so, we would lose the date calculation possibilities that MySQL offers. Among those are, finding the week day from a date, or determining the date that falls thirty days after a certain date. So for the date (and time), a single column can handle it all, although at the UI level, separate entry fields should be displayed for year, month, and day. This is to avoid any possibility of mix-up and also because we cannot expect users to know about what MySQL accepts as a valid date. There is a certain latitude in the range of valid values but we can take it for granted that users have unlimited creativity, regarding how to enter invalid values. If a single field is present on the UI, clear directions should be provided to help with filling this field correctly. Data Elements Containing Formatting Characters The last case we’ll examine is the phone number. In many parts of the world, the phone number follows a specific pattern and also uses formatting characters for legibility. In North America, we have a regional code, an exchange number, and phone number, for example, 418-111-2222; an extension could possibly be appended to the phone number. However, in practice only the regional code and extension are separated from the rest into data elements of their own. Moreover, people often enter formatting characters like (418) 111-2222 and expect those to be output back. So, a standard output format must be chosen, and then the correct number of sub-elements will have to be set into the model to be able to recreate the expected output. Data that are Results Even though it might seem natural to have a distinct element for the total_price of the car, in practice this is not justified. The reason is that the total price is a computed result. Having the total price printed on a sales contract constitutes an output. Thus, we eliminate this information in the list of column names. For the same reason, we could omit the tax column because it can be computed.
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Data Naming [ 28 ] At this point, a consensus must be reached by convincing users about the importance of standard terms. It must become clear to everyone that the term car_number is not precise enough to be used, so it will be replaced by car_internal_number …….in the data elements list, probably also in any user interface (UI) or report. It can be argued that car_internal_number should be replaced by something more appropriate; the important point here is we merged two synonyms: car_number and stock_number,……………………………………………………….. and established the difference between two elements that looked similar but were not, eliminating a source of confusion. Therefore we end up with the following elements: Car_serial_number Car_internal_number (former car id number and stock number) Eventually, when dealing with data grouping, another decision will have to be taken: to which number, serial or internal, do we associate the car’s physical key number. Subdividing Data Elements In this section, we try to find out if some elements should be broken into more simple ones. The reason for doing so is that, if an element is composed of many parts, applications will have to break it for sorting and selection purposes. Thus it’s better to break the elements right now at the source. Recomposing it will be easier at the application level. Breaking the elements provides more clarity at the UI level. Therefore, at this level we will avoid (as much as possible) the well-known last-name/first-name inversion problem. As an example for this problem, let’s take the buyer’s name. During the interview, we noticed that the name is expressed in various ways on the forms: Form How the name is expressed Delivery certificate Mr Joe Smith Sales contract Smith, Joe We notice that There is a salutation element, Mr The element name is too imprecise; we really have a first name and a last name On the sales contract, the comma after our last name should really be excluded from the element, as it’s only a formatting character
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