In this chapter, we will focus on using SQL to create the database and table structures, In Chapter 16, we will use SQL as a data manipulation language ( DML) to insert, delete, . It can contain search conditions similar to a WHERE clause. Create the table shown here in SQL Server and show the statements you used. You can also insert a valid value to an AUTO_INCREMENT column, Join via WHERE clause (lagacy and not recommended) mysql> SELECT boutiquedababr3.xyz, .. WHERE lastVisitDate BETWEEN '' AND CURDATE() ORDER BY record (customer_id) changes, update the matching records here FOREIGN. If you aren't familiar with IF Statements, click here to read that Just like the AND function, you can have up to 30 OR conditions In the embedded Excel workbook below insert a formula (in the grey .. Before entering the data under Product Unit Price, use a policy to Value if TEST is False = F3 *
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There are a couple of ways you can load lots of data quickly. The naive approach is to simply call Model. You can get a significant speedup by simply wrapping this in a transaction with atomic.
The above code still suffers from points 2, 3 and 4. This method accepts a list of tuples or dictionaries, and inserts multiple rows in a single query:. SQLite users should be aware of some caveats when using bulk inserts.
Specifically, your SQLite3 version must be 3. Depending on the number of rows in your data source, you may need to break it up into chunks. You can write a loop to batch your data into chunks in which case it is strongly recommended you use a transaction:. Peewee comes with a chunked helper function which you can use for efficiently chunking a generic iterable into a series of batch -sized iterables:.
Alternatively, you can use the Database. This method also provides a workaround for databases besides Postgresql, when the primary-key of the newly-created rows must be obtained. The following example will update all Tweet objects, marking them as published , if they were created before today.
For more information, see the documentation on Model. If you would like more information on performing atomic updates such as incrementing the value of a column , check out the atomic update recipes. Peewee allows you to perform atomic updates. The naive approach would be to write something like this:. Do not do this! Not only is this slow, but it is also vulnerable to race conditions if multiple processes are updating the counter at the same time.
Instead, you can update the counters atomically using update:. You can make these update statements as complex as you like. We can even use a subquery to update the value of a column. Suppose we had a denormalized column on the User model that stored the number of tweets a user had made, and we updated this value periodically. Here is how you might write such a query:. Peewee provides support for varying types of upsert functionality. With SQLite prior to 3.
In the above example, we could safely invoke the upsert query as many times as we wanted. The login count will be incremented atomically, the last login column will be updated, and no duplicate rows will be created. Postgresql and SQLite 3. For more information, see Insert.
To delete a single model instance, you can use the Model. The following will delete all Tweet objects that are over one year old:. You can use the Model. For primary-key lookups, you can also use the shortcut method Model.
This method is a shortcut that calls Model. Additionally, if no model matches the given query, a DoesNotExist exception will be raised. For more advanced operations, you can use SelectBase. The following query retrieves the latest tweet from the user named charlie:.
Failing that, a new row will be created. You can easily encapsulate this type of logic as a classmethod on your own Model classes. The above example first attempts at creation, then falls back to retrieval, relying on the database to enforce a unique constraint. This method is implemented along the same lines as the Django function of the same name. The function returns a 2-tuple containing the instance and a boolean value indicating if the object was created. Suppose we have a different model Person and would like to get or create a person object.
The only conditions we care about when retrieving the Person are their first and last names, but if we end up needing to create a new record, we will also specify their date-of-birth and favorite color:. For more details read the documentation for Model. We can use Model.
Peewee allows you to iterate over these rows, as well as use indexing and slicing operations:. Select queries are smart, in that you can iterate, index and slice the query multiple times but the query is only executed once.
In the following example, we will simply call select and iterate over the return value, which is an instance of Select. This will return all the rows in the User table:. Subsequent iterations of the same query will not hit the database as the results are cached. To disable this behavior to reduce memory usage , call Select.
When iterating over a model that contains a foreign key, be careful with the way you access values on related models. When you create a foreign key, such as Tweet. Back-references are exposed as Select instances:. You can iterate over the user. In addition to returning model instances, Select queries can return dictionaries, tuples and namedtuples. Depending on your use-case, you may find it easier to work with rows as dictionaries, for example:.
See namedtuples , tuples , dicts for more information. By default peewee will cache the rows returned when iterating over a Select query. This is an optimization to allow multiple iterations as well as indexing and slicing without causing additional queries. This caching can problematic, however, when you plan to iterate over a large number of rows. To reduce the amount of memory used by peewee when iterating over a query, use the iterator method.
This method allows you to iterate without caching each model returned, using much less memory when iterating over large result sets. For simple queries you can see further speed improvements by returning rows as dictionaries, namedtuples or tuples. The following methods can be used on any Select query to change the result row type:. For example, the above code might look like:. When iterating over a large number of rows that contain columns from multiple tables, peewee will reconstruct the model graph for each row returned.
This operation can be slow for complex graphs. In addition to the above row-types, there is a fourth method objects which will return the rows as model instances, but will not attempt to resolve the model graph.
For maximum performance, you can execute queries and then iterate over the results using the underlying database cursor. The cursor will return the raw row-tuples:. You can filter for particular records using normal python operators.
Peewee supports a wide variety of query operators. The reason for this is that Python coerces the return value of logical operations to a boolean value. Check out the table of query operations to see what types of queries are possible.
For a wide range of example queries, see the Query Examples document, which shows how to implements queries from the PostgreSQL Exercises website. You can also order across joins. When sorting on a calculated value, you can either include the necessary SQL expressions, or reference the alias assigned to the value. Here are two examples illustrating these methods:. Alternatively, you can reference the alias assigned to the calculated value in the select clause.
This method has the benefit of being a bit easier to read. Note that we are not referring to the named alias directly, but are wrapping it using the SQL helper:. Occasionally you may want to pull a random record from the database. You can accomplish this by ordering by the random or rand function depending on your database:.
The paginate method makes it easy to grab a page or records. If you would like more granular control, you can always use limit and offset. Suppose you have some users and want to get a list of them along with the count of tweets in each. The resulting query will return User objects with all their normal attributes plus an additional attribute count which will contain the count of tweets for each user.
We use a left outer join to include users who have no tweets. You can retrieve scalar values by calling Query. A Window function refers to an aggregate function that operates on a sliding window of data that is being processed as part of a SELECT query. Window functions make it possible to do things like:. 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. 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: 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. Both products and suppliers tables exhibit a one-to-many relationship to the junction table. The many-to-many relationship is supported via the junction table. Suppose that some products have optional data e.
Before we conclude this example, let's run the mysqldump utility program to dump out backup the entire southwind database. You can restore from the backup by running the script either:. In the relational model, a table shall not contain duplicate rows, because that would create ambiguity in retrieval. To ensure uniqueness, each table should have a column or a set of columns , called primary key , that uniquely identifies every record of the table.
For example, an unique number customerID can be used as the primary key for the customers table; productCode for products table; isbn for books table. A primary key is called a simple key if it is a single column; it is called a composite key if it is made up of several columns. The primary key is often used to relate to other tables.
A foreign key of a child table is used to reference the parent table. Foreign key constraint can be imposed to ensure so-called referential integrity - values in the child table must be valid values in the parent table. We define the foreign key when defining the child table, which references a parent table, as follows:. Indexes or Keys can be created on selected column s to facilitate fast search. If productID column is indexed e.
The drawback about indexing is cost and space. Building and maintaining indexes require computations and memory spaces. Nevertheless, relational databases are typically optimized for queries and retrievals, but NOT for updates. There can be more than one indexes in a table.
Index are automatically built on the primary-key column s. Results of one query can be used in another SQL statement. Subquery is useful if more than one tables are involved. In the previous many-to-many product sales example, how to find the suppliers that do not supply any product?
A subquery may return a scalar, a single column, a single row, or a table. You can use comparison operator e. Date and time are of particular interest for database applications. Let's begin with Date without Time with the following example. Take note that date value must be written as a string in the format of 'yyyy-mm-dd' , e. MySQL provides these built-in functions for getting the current date, time and datetime:.
MySQL will issue a warning and insert all zeros e. A view is a virtual table that contains no physical data. It provide an alternative way to look at the data. Transaction is important to ensure that there is no partial update to the database, given an atomic of SQL statements. If you start another mysql client and do a SELECT during the transaction before the commit or rollback , you will not see the changes.
Alternatively, you can also disable the so-called autocommit mode, which is set by default and commit every single SQL statement. In an inner join of two tables, each row of the first table is combined joined with every row of second table. This is important, as in some queries, you are interested to have result on every row on the left table, with no match in the right table, e. Peter runs a small car rental company with 10 cars and 5 trucks. He engages you to design a web portal to put his operation online.
A vehicle, identified by the vehicle registration number, can be rented on a daily basis. The rental rate is different for different vehicles.
A customer can rental a vehicle from a start date to an end date. The initial database contains 3 tables: We shall begin with the first table called " products " with the following columns having data types as indicated and rows: We use fixed-length string for productCode , as we assume that the productCode contains exactly 3 characters.
To insert a row with values on selected columns only, use: Arithmetic Operators You can perform arithmetic operations on numeric fields using arithmetic operators, as tabulated below: NULL cannot be compared. SET command, with the following syntax: Use with extreme care! Records are NOT recoverable!!!
You need to provide the path absolute or relative and the filename. If you use another delimiter, e. For example, to restore the southwind backup earlier: One-To-Many Relationship Suppose that each product has one supplier, and each supplier supplies one or more products. 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: Many-To-Many Relationship Suppose that a product has many suppliers; and a supplier supplies many products in a so-called many-to-many relationship.
One-to-one Relationship Suppose that some products have optional data e. You can restore from the backup by running the script either: Foreign Key A foreign key of a child table is used to reference the parent table. We define the foreign key when defining the child table, which references a parent table, as follows: Cannot delete or update a parent row: Indexes or Keys Indexes or Keys can be created on selected column s to facilitate fast search.
In MySQL, indexes can be built on: The valid range is ' You can set a value using the valid format e.
Ultimate List of 40 Important SQL Queries
Here are a few of the most important ones – each of these queries is If you want to display all the attributes from a particular table, this is the right query to use: This query updates the view named 'Product List' – and if this view doesn't exist, In our first SQL example, imagine a situation where the zip and phone fields. For example, if a user answers "Did you use our product?" with "Yes Here are some guides related to this conditional logic option: How to. Make sure that the fields are set to , meaning their value is numeric. You can also insert your own formula here, you can see the number pad.