Periodic Table 1.1 serial key or number

Using Zend_Db_Table as a concrete class

Declare the database table for which this class is defined, using the protected variable . This is a string, and must contain the name of the table spelled as it appears in the database.

If you don't specify the table name, it defaults to the name of the class. If you rely on this default, the class name must match the spelling of the table name as it appears in the database.

You can also declare the schema for the table, either with the protected variable , or with the schema prepended to the table name in the property. Any schema specified with the property takes precedence over a schema specified with the property. In some brands, the term for schema is "database" or "tablespace," but it is used similarly.

The schema and table names may also be specified via constructor configuration directives, which override any default values specified with the and properties. A schema specification given with the name directive overrides any value provided with the schema option.

If you don't specify the schema name, it defaults to the schema to which your database adapter instance is connected.

Every table must have a primary key. You can declare the column for the primary key using the protected variable . This is either a string that names the single column for the primary key, or else it is an array of column names if your primary key is a compound key.

If you don't specify the primary key, Zend_Db_Table_Abstract tries to discover the primary key based on the information provided by the describeTable()´ method.

Note: Every table class must know which columns can be used to address rows uniquely. If no primary key columns are specified in the table class definition or the table constructor arguments, or discovered in the table metadata provided by describeTable(), then the table cannot be used with Zend_Db_Table.

When you create an instance of a Table class, the constructor calls a set of protected methods that initialize metadata for the table. You can extend any of these methods to define metadata explicitly. Remember to call the method of the same name in the parent class at the end of your method.

The setup methods you can override are the following:

• _setupDatabaseAdapter() checks that an adapter has been provided; gets a default adapter from the registry if needed. By overriding this method, you can set a database adapter from some other source.

• _setupTableName() defaults the table name to the name of the class. By overriding this method, you can set the table name before this default behavior runs.

• _setupMetadata() sets the schema if the table name contains the pattern "schema.table"; calls describeTable() to get metadata information; defaults the array to the columns reported by describeTable(). By overriding this method, you can specify the columns.

• _setupPrimaryKey() defaults the primary key columns to those reported by describeTable(); checks that the primary key columns are included in the array. By overriding this method, you can specify the primary key columns.

If application-specific logic needs to be initialized when a Table class is constructed, you can select to move your tasks to the init() method, which is called after all Table metadata has been processed. This is recommended over the __construct() method if you do not need to alter the metadata in any programmatic way.

Creating an Instance of a Table

The first way to provide a database adapter to a Table class is by passing it as an object of type Zend_Db_Adapter_Abstract in the options array, identified by the key 'db'.

The second way to provide a database adapter to a Table class is by declaring an object of type Zend_Db_Adapter_Abstract to be a default database adapter for all subsequent instances of Tables in your application. You can do this with the static method Zend_Db_Table_Abstract::setDefaultAdapter(). The argument is an object of type Zend_Db_Adapter_Abstract.

It can be convenient to create the database adapter object in a central place of your application, such as the bootstrap, and then store it as the default adapter. This gives you a means to ensure that the adapter instance is the same throughout your application. However, setting a default adapter is limited to a single adapter instance.

The third way to provide a database adapter to a Table class is by passing a string in the options array, also identified by the 'db' key. The string is used as a key to the static Zend_Registry instance, where the entry at that key is an object of type Zend_Db_Adapter_Abstract.

Like setting the default adapter, this gives you the means to ensure that the same adapter instance is used throughout your application. Using the registry is more flexible, because you can store more than one adapter instance. A given adapter instance is specific to a certain brand and database instance. If your application needs access to multiple databases or even multiple database brands, then you need to use multiple adapters.

Inserting Rows to a Table

An auto-incrementing primary key generates a unique integer value for you if you omit the primary key column from your statement.

In Zend_Db_Table_Abstract, if you define the protected variable to be the Boolean value , then the class assumes that the table has an auto-incrementing primary key.

MySQL, Microsoft Server, and SQLite are examples of brands that support auto-incrementing primary keys.

PostgreSQL has a notation that implicitly defines a sequence based on the table and column name, and uses the sequence to generate key values for new rows. has an notation that works similarly. If you use either of these notations, treat your Zend_Db_Table class as having an auto-incrementing column with respect to declaring the member as .

A sequence is a database object that generates a unique value, which can be used as a primary key value in one or more tables of the database.

If you define to be a string, then Zend_Db_Table_Abstract assumes the string to name a sequence object in the database. The sequence is invoked to generate a new value, and this value is used in the operation.

Oracle, PostgreSQL, and are examples of brands that support sequence objects in the database.

PostgreSQL and also have syntax that defines sequences implicitly and associated with columns. If you use this notation, treat the table as having an auto-incrementing key column. Define the sequence name as a string only in cases where you would invoke the sequence explicitly to get the next key value.

Some tables have a natural key. This means that the key is not automatically generated by the table or by a sequence. You must specify the value for the primary key in this case.

If you define the to be the Boolean value , then Zend_Db_Table_Abstract assumes that the table has a natural primary key. You must provide values for the primary key columns in the array of data to the insert() method, or else this method throws a Zend_Db_Table_Exception.

Note: All brands support tables with natural keys. Examples of tables that are often declared as having natural keys are lookup tables, intersection tables in many-to-many relationships, or most tables with compound primary keys.

Deleting Rows from a Table

Finding Rows by Primary Key

Querying for a Set of Rows

The Zend_Db_Table_Select object is an extension of the Zend_Db_Select object that applies specific restrictions to a query. The enhancements and restrictions are:

Protons and neutrons

The nucleus is itself composed of two kinds of particles. are the carriers of positive electric charge in the nucleus; the proton charge is exactly the same as the electron charge, but of opposite sign. This means that in any [electrically neutral] atom, the number of protons in the nucleus (often referred to as the nuclear charge) is balanced by the same number of electrons outside the nucleus.

Because the electrons of an atom are in contact with the outside world, it is possible for one or more electrons to be lost, or some new ones to be added. The resulting electrically-charged atom is called an .

The other nuclear particle is the . As its name implies, this particle carries no electrical charge. Its mass is almost the same as that of the proton. Most nuclei contain roughly equal numbers of neutrons and protons, so we can say that these two particles together account for almost all the mass of the atom.

Atomic Number (Z)

What single parameter uniquely characterizes the atom of a given element? It is not the atom's relative mass, as we will see in the section on isotopes below. It is, rather, the number of protons in the nucleus, which we call the and denote by the symbol Z. Each proton carries an electric charge of +1, so the atomic number also specifies the electric charge of the nucleus. In the neutral atom, the Z protons within the nucleus are balanced by Zelectrons outside it.

Atomic numbers were first worked out in 1913 by Henry Moseley, a young member of Rutherford's research group in Manchester.

Moseley searched for a measurable property of each element that increases linearly with atomic number. He found this in a class of X-rays emitted by an element when it is bombarded with electrons. The frequencies of these X-rays are unique to each element, and they increase uniformly in successive elements. Moseley found that the square roots of these frequencies give a straight line when plotted against Z; this enabled him to sort the elements in order of increasing atomic number.

You can think of the atomic number as a kind of serial number of an element, commencing at 1 for hydrogen and increasing by one for each successive element. The chemical name of the element and its symbol are uniquely tied to the atomic number; thus the symbol "Sr" stands for strontium, whose atoms all have Z = 38.

Mass number (A)

This is just the sum of the numbers of protons and neutrons in the nucleus. It is sometimes represented by the symbol A, so

in which Z is the atomic number and N is the neutron number.

Nuclides and their Symbols

The term simply refers to any particular kind of nucleus. For example, a nucleus of atomic number 7 is a nuclide of nitrogen. Any nuclide is characterized by the pair of numbers (Z ,A). The element symbol depends on Z alone, so the symbol ²⁶Mg is used to specify the mass-26 nuclide of manganese, whose name implies Z=12. A more explicit way of denoting a particular kind of nucleus is to add the atomic number as a subscript. Of course, this is somewhat redundant, since the symbol Mg always implies Z=12, but it is sometimes a convenience when discussing several nuclides.

Two nuclides having the same atomic number but different mass numbers are known as . Most elements occur in nature as mixtures of isotopes, but twenty-three of them (including beryllium and fluorine, shown in the table) are monoisotopic. For example, there are three natural isotopes of magnesium: ²⁴Mg (79% of all Mg atoms), ²⁵Mg (10%), and ²⁶Mg (11%); all three are present in all compounds of magnesium in about these same proportions.

Approximately 290 isotopes occur in nature. The two heavy isotopes of hydrogen are especially important— so much so that they have names and symbols of their own:

Deuterium accounts for only about 15 out of every one million atoms of hydrogen. Tritium, which is radioactive, is even less abundant. All the tritium on the earth is a by-product of the decay of other radioactive elements.

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