Mysql drop database table relationship

How to Create a Database from a Script in MySQL | wagtailfarm.info

mysql drop database table relationship

In this database model, each building has one or more rooms. The relationship between the buildings and rooms tables is one-to-many (1:N) as illustrated in. Access database: mysql -u [username] -p [database] (will prompt for password) Delete all records from a table (without dropping the table itself): DELETE. It is recommended that you make a backup before you perform any destructive actions, such as dropping database tables. Without a backup, there will be no.

To save time and typing, readers are encouraged to copy the green entries from the example boxes and paste them into your MySQL command-line session: In this display, drag your mouse cursor across the phrase you want to capture, highlighting it. In the MySQL command-line window, select "Paste" how to do that unfortunately depends on the platform.

Okay, let's add two more records to our table: Frank Wright 37 Red. Seymour Hawthorne 82 None of your business. If something goes wrong and you want to start over entering records into the table, you may want to delete all existing records first. And please notice that "delete from trivia" note the bold word deletes the records and keeps the table, but "delete trivia" deletes the table and its data.

At this point, let's assume we have successfully entered three records into the table. Now we can perform our first meaningful query: The reason he didn't is because MySQL sorted the names just as we entered them — as a single string composed of first name, then last name. At this point it may occur to the reader that name data should always be entered into separate fields for first and last name.

The underlying idea is that one can always combine a first and last name to create a full name, but one cannot easily do the reverse. Split a Field in Two, Make a New Table This is slightly more ambitious — we're going to create a new table with separate fields for first and last name, then we're going to transfer all the records from the original table into the new one.

The point of this exercise is to show how to do this without having to re-enter the records.

mysql drop database table relationship

As a first step, before creating a new table, let's see if we can successfully split the "Name" field into first and last names: Now let's create a new table with the names split in the same way: This isn't always necessary — in many cases one can write a query for an existing table to get different results without having to create a new table each time.

Bruce Callow 13 I haven't decided yet. Advanced Database Queries 1: People For more advanced query practice, I decided a table with only three records wasn't adequate, so I've created a much larger table composed of records describing make-believe people. Readers have two ways to get this table into their tutorial database: The first of the two options above is simpler, and is preferred.

Here is how to proceed: Download the file to any convenient location. Make a copy of the file in a directory that's easily located by the MySQL command-line application. From the MySQL application, type this: In this table's description, we see there's a field named "pk" with a new role we haven't seen before — as a "primary key". As tables become larger, the presence of a unique primary key becomes more important for a number of reasons.

One, MySQL can read from and write to the table more efficiently. Two, it makes each record unique — without a unique key, two records with identical field data would not be distinct MySQL would not be able to tell them apart. There's another, even more important role for table keys in the design of relational databases, a topic we'll be getting into later.

Learning MySQL and MariaDB by Russell J.T. Dyer

But back to our new table — with records, we can pose some interesting queries. Here are some examples. Let's say you're responsible for a company database, and your boss shows up and asks, "How many of our customers are a interested in rock music, and b age 20 and younger? Being fully versed in modern database technology, you create an answer for him in a matter of seconds: Because I generated the "people" table using a seeded pseudorandom number generator, the numbers you acquire from your invocation of the "people" table should agree with those here.

This also allows you to compare your results with the original ones, as an accuracy check for your query entries.

The boss thinks for a minute and asks, "Can you show me the breakdown by musical preference for that age group? ZIP Codes The above exercise used a fairly large database of records. But this example uses a much larger U. ZIP code database having over 43, records. I include this query example because it shows that one can extract many interesting facts from a large table with many fields.

We will proceed as in the previous example, but with one possible change — this database is so large that users may elect to download a zipped version of the file: This will yield a file named "zipcodes. Those with fast Internet connections may prefer to downlaod the original, uncompressed archive zipcodes.

How to drop all tables in MySQL? | TablePlus

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Creating Databases and Tables In order to be able to add and manipulate data, you first have to create a database. Creating a table is more involved and offers many choices. There are several types of tables from which to choose, some with unique features. When creating tables, you must also decide on the structure of each table: There are a few basic things to decide when creating a structure for your data: This is a good starting point for creating a database and tables.

As we get further along, we can expand that list of column data types to improve the performance of your databases. This chapter contains examples of how to create a database and tables. The text is written on the assumption that you will enter the SQL statements shown on your server, using the mysql client. The exercises at the end of this chapter will require that you make some changes and additions to the database and its tables on your computer. So, when instructed, be sure to try all of the examples on your computer.

MySQL ON DELETE CASCADE: Deleting Data From Multiple Related Tables

In those later chapters, you will be asked to add, retrieve, and change data from the tables you create in this chapter. Exercises in subsequent chapters assume that you have created the tables you are asked to create in this chapter. It will help reinforce what you read, and you will learn more.

You will have to provide a name for the database with this SQL statement. You could call it something bland like db1.

mysql drop database table relationship

Some birds live in groups, or a colony called a rookery. To do this, enter the following from within the mysql client: It will just set up a place to add tables, which will in turn hold data.

The results are the same. You can, though, do a bit more than the SQL statement shown here for creating a database. You can add a couple of options in which you can set the default types of characters that will be used in the database and how data will be sorted or collated. The second line, which is new, tells MySQL that the default characters that will be used in tables in the database are Latin letters and other characters. The third line tells MySQL that the default method of sorting data in tables is based on binary Latin characters.

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In fact, for most purposes, the minimal method of creating a database without options, as shown earlier, is fine. You can always change these two options later if necessary. To get a list of databases, enter the following SQL statement: Before beginning to add tables to the rookery database, enter the following command into the mysql client: USE rookery This little command will set the new database that was just created as the default database for the mysql client.

It will remain the default database until you change it to a different one or until you exit the client. Otherwise, when you enter each table-related SQL statement, you would have to specify each time the database where the table is located. Creating Tables The next step for structuring a database is to create tables. The main table will have a bunch of columns, but the reference tables will have only a few columns. For our fictitious bird-watchers site, the key interest is birds.

So we want to create a table that will hold basic data on birds. Enter the following SQL statement into mysql on your computer: Note that all the columns together are contained in a pair of parentheses. For each colum, we specify the name, the type, and optional settings. For instance, the information we give about the first column is: Actually, you can use a reserve word, but it must always be given within quotes to distinguish it.

We created this table with only five columns. It will start with the number 1, unless we specify a different number. The next column will contain the scientific name of each bird e. But the scientific name can be very long and usually in Latin or Greek or sometimes a mix of both languagesand not everyone is comfortable using words from these languages.

In addition, would be awkward to enter the scientific name of a bird when referencing a row in the table. If the scientific name of a bird has fewer than characters, the storage engine will reduce the size of the column for the row. This is different from the CHAR column data type. If the data in a CHAR column is less than its maximum, space is still allocated for the full width that you set.

There are trade-offs with these two basic character data types. If the storage engine knows exactly what to expect from a column, tables run faster and can be indexed more easily with a CHAR column. That can improve performance. When you know for sure that a column will have a set number of characters, use CHAR. They are integer data types i.

Then, when manipulating data, we can join the two tables, use a number to identify each family, and link each bird to its family. The last column is for the description of each bird. This will allow us to enter plenty of text about each bird. We could write multiple pages describing a bird and put it in this column. There are additional factors to consider when searching for a bird in a database, so there are many columns we could add to this table: In addition, there are many other data types that may be used for columns.

We can have columns that allow for larger and smaller numbers, or for binary files to be included in each row. For instance, you might want a column with a binary data type to store a photograph of each bird. However, this basic table gives you a good sampling of the possibilities when creating tables. It displays information about the columns of a table, or the table schema—not the data itself. To use this SQL statement to get information on the table we just created, you would enter the following SQL statement: The first row of this results set contains column headings describing the rows of information that follow it.

In the first column of this results set, Field contains the fields or columns of the table created. The second column, Type, lists the data type for each field.

The third column in the preceding results, Null, indicates whether each field may contain NULL values. This is different from blank or empty content in a field.

That may seem strange: The fourth column, Key, indicates whether a field is a key field—an indexed column. If a column is indexed, the display will say which kind of index. Because of the limited space permitted in the display, it truncates the words.