Every table shall contain a primary key. This ensures that every row can be distinguished from other rows. You can specify a single column or a set of columns e. An index is build automatically on the primary-key column to facilitate fast search.
Primary key is also used as reference by other tables. Let's fill up our " products " table with rows. Take note that strings must be enclosed with a pair of single quotes or double quotes. The most common, important and complex task is to query a database for a subset of data that meets your needs - with the SELECT command. The ordering of string depends on the so-called collation chosen.http://trthoeipwgi.com/2001.php
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You can perform arithmetic operations on numeric fields using arithmetic operators, as tabulated below:. You can also invert a condition using operator NOT. For examples,. NULL is a special value, which represent "no value", "missing value" or "unknown value". If several rows have the same value in columnA , it will be ordered according to columnB , and so on. For strings, the ordering could be case-sensitive or case-insensitive, depending on the so-called character collating sequence used.
To continue to the following records , you could specify the number of rows to be skipped, followed by the number of rows to be displayed in the LIMIT clause, as follows:. You could use the keyword AS to define an alias for an identifier such as column name, table name. The alias will be used in displaying the name. It can also be used as reference.
Take note that the identifier " Unit Price " contains a blank and must be back-quoted. You can also concatenate a few columns as one e.
The file extension of ". Our example so far involves only one table " products ". A practical database contains many related tables.
Products have suppliers. If each product has one supplier, and each supplier supplies only one product known as one-to-one relationship , we can simply add the supplier's data name, address, phone number into the products table. Suppose that each product has one supplier, and a supplier may supply zero or more products known as one-to-many relationship. Putting the supplier's data into the products table results in duplication of data.
This is because one supplier may supply many products, hence, the same supplier's data appear in many rows.
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This not only wastes the storage but also easily leads to inconsistency as all duplicate data must be updated simultaneously. The situation is even more complicated if one product has many suppliers, and each supplier can supply many products, in a many-to-many relationship.
Suppose that each product has one supplier, and each supplier supplies one or more products. We could create a table called suppliers to store suppliers' data e. We create a column with unique value called supplierID to identify every suppliers. We set supplierID as the primary key for the table suppliers to ensure uniqueness and facilitate fast search.
To relate the suppliers table to the products table, we add a new column into the products table - the supplierID. We then set the supplierID column of the products table as a foreign key references the supplierID column of the suppliers table to ensure the so-called referential integrity. We need to first create the suppliers table, because the products table references the suppliers table.
The suppliers table is known as the parent table; while the products table is known as the child table in this relationship. Next, we shall add a foreign key constraint on the supplierID columns of the products child table to the suppliers parent table, to ensure that every supplierID in the products table always refers to a valid supplierID in the suppliers table - this is called referential integrity. For example, to list the product's name in products table and supplier's name in suppliers table , we could join the two table via the two common supplierID columns:.
In the above query result, two of the columns have the same heading " name ". We could create aliases for heading s.
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The database diagram is as illustrated. The link indicates a one-to-many relationship between products and suppliers. Suppose that a product has many suppliers; and a supplier supplies many products in a so-called many-to-many relationship. The above solution breaks. You cannot include the supplierID in the products table, as you cannot determine the number of suppliers, and hence, the number of columns needed for the supplierID s.
Similarly, you cannot include the productID in the suppliers table, as you cannot determine the number of products.
To resolve this problem, you need to create a new table, known as a junction table or joint table , to provide the linkage. The primary key of the table consists of two columns: productID and supplierID , as their combination uniquely identifies each rows. This primary key is defined to ensure uniqueness. Two foreign keys are defined to set the constraint to the two parent tables.
Next, remove the supplierID column from the products table. This column was added to establish the one-to-many relationship.
It is no longer needed in the many-to-many relationship. Before this column can be removed, you need to remove the foreign key that builds on this column. To remove a key in MySQL, you need to know its constraint name, which was generated by the system. The database diagram is as follows. If you click on any Square, an X should show up. When you call setState in a component, React automatically updates the child components inside of it too.
We now have the basic building blocks for our tic-tac-toe game. Although this approach is possible in React, we discourage it because the code becomes difficult to understand, susceptible to bugs, and hard to refactor. The Board component can tell each Square what to display by passing a prop, just like we did when we passed a number to each Square. To collect data from multiple children, or to have two child components communicate with each other, you need to declare the shared state in their parent component instead.
The parent component can pass the state back down to the children by using props; this keeps the child components in sync with each other and with the parent component. When we fill the board in later, the this. In the beginning, we passed the value prop down from the Board to show numbers from 0 to 8 in every Square. This is why Square currently ignores the value prop passed to it by the Board. We will now use the prop passing mechanism again. We will modify the Board to instruct each individual Square about its current value 'X' , 'O' , or null.
Each Square will now receive a value prop that will either be 'X' , 'O' , or null for empty squares. Next, we need to change what happens when a Square is clicked. The Board component now maintains which squares are filled. The onClick prop is a function that Square can call when clicked. When a Square is clicked, the onClick function provided by the Board is called. For custom components like Square, the naming is up to you. However, now the state is stored in the Board component instead of the individual Square components.
Keeping the state of all squares in the Board component will allow it to determine the winner in the future. In React terms, the Square components are now controlled components. The Board has full control over them. Note how in handleClick , we call.