Postgres pro _ documentation_ 9.5_ constraints _ postgres professional database 4 net
A check constraint is the most generic constraint type. Data recovery galaxy s5 It allows you to specify that the value in a certain column must satisfy a Boolean (truth-value) expression. Data recovery galaxy s6 For instance, to require positive product prices, you could use: CREATE TABLE products (
As you see, the constraint definition comes after the data type, just like default value definitions. Data recovery google store Default values and constraints can be listed in any order. Data recovery galaxy s4 A check constraint consists of the key word CHECK followed by an expression in parentheses. Moto g data recovery The check constraint expression should involve the column thus constrained, otherwise the constraint would not make too much sense.
You can also give the constraint a separate name. Moto g data recovery software This clarifies error messages and allows you to refer to the constraint when you need to change it. Data recovery houston The syntax is: CREATE TABLE products (
So, to specify a named constraint, use the key word CONSTRAINT followed by an identifier followed by the constraint definition. Data recovery hard drive cost (If you don’t specify a constraint name in this way, the system chooses a name for you.)
A check constraint can also refer to several columns. Data recovery hardware tools Say you store a regular price and a discounted price, and you want to ensure that the discounted price is lower than the regular price: CREATE TABLE products (
The first two constraints should look familiar. Data recovery hard disk The third one uses a new syntax. Data recovery houston tx It is not attached to a particular column, instead it appears as a separate item in the comma-separated column list. Data recovery hard drive software Column definitions and these constraint definitions can be listed in mixed order.
We say that the first two constraints are column constraints, whereas the third one is a table constraint because it is written separately from any one column definition. Hdata recovery master Column constraints can also be written as table constraints, while the reverse is not necessarily possible, since a column constraint is supposed to refer to only the column it is attached to. H data recovery registration code ( Postgres Pro doesn’t enforce that rule, but you should follow it if you want your table definitions to work with other database systems.) The above example could also be written as: CREATE TABLE products (
It should be noted that a check constraint is satisfied if the check expression evaluates to true or the null value. H data recovery software Since most expressions will evaluate to the null value if any operand is null, they will not prevent null values in the constrained columns. H data recovery registration code free download To ensure that a column does not contain null values, the not-null constraint described in the next section can be used.
Unique constraints ensure that the data contained in a column, or a group of columns, is unique among all the rows in the table. Data recovery iphone The syntax is: CREATE TABLE products (
To define a unique constraint for a group of columns, write it as a table constraint with the column names separated by commas: CREATE TABLE example (
This specifies that the combination of values in the indicated columns is unique across the whole table, though any one of the columns need not be (and ordinarily isn’t) unique.
Adding a unique constraint will automatically create a unique B-tree index on the column or group of columns listed in the constraint. Data recovery iphone 6 A uniqueness restriction covering only some rows cannot be written as a unique constraint, but it is possible to enforce such a restriction by creating a unique partial index.
In general, a unique constraint is violated if there is more than one row in the table where the values of all of the columns included in the constraint are equal. Data recovery iphone 5 However, two null values are never considered equal in this comparison. Data recovery ipad That means even in the presence of a unique constraint it is possible to store duplicate rows that contain a null value in at least one of the constrained columns. Data recovery images This behavior conforms to the SQL standard, but we have heard that other SQL databases might not follow this rule. Data recovery iso So be careful when developing applications that are intended to be portable.
A primary key constraint indicates that a column, or group of columns, can be used as a unique identifier for rows in the table. Data recovery icon This requires that the values be both unique and not null. Data recovery illustrator So, the following two table definitions accept the same data: CREATE TABLE products (
Adding a primary key will automatically create a unique B-tree index on the column or group of columns listed in the primary key, and will force the column(s) to be marked NOT NULL.
A table can have at most one primary key. Data recovery iphone free (There can be any number of unique and not-null constraints, which are functionally almost the same thing, but only one can be identified as the primary key.) Relational database theory dictates that every table must have a primary key. Data recovery ios This rule is not enforced by Postgres Pro, but it is usually best to follow it.
Primary keys are useful both for documentation purposes and for client applications. I data recovery software free download For example, a GUI application that allows modifying row values probably needs to know the primary key of a table to be able to identify rows uniquely. Icare data recovery There are also various ways in which the database system makes use of a primary key if one has been declared; for example, the primary key defines the default target column(s) for foreign keys referencing its table.
A foreign key constraint specifies that the values in a column (or a group of columns) must match the values appearing in some row of another table. I phone data recovery We say this maintains the referential integrity between two related tables.
Let’s also assume you have a table storing orders of those products. I card data recovery We want to ensure that the orders table only contains orders of products that actually exist. Data recovery jobs So we define a foreign key constraint in the orders table that references the products table: CREATE TABLE orders (
We say that in this situation the orders table is the referencing table and the products table is the referenced table. Data recovery jacksonville fl Similarly, there are referencing and referenced columns.
A foreign key can also constrain and reference a group of columns. Data recovery johannesburg As usual, it then needs to be written in table constraint form. Data recovery jakarta Here is a contrived syntax example: CREATE TABLE t1 (
A table can have more than one foreign key constraint. Data recovery jaipur This is used to implement many-to-many relationships between tables. Data recovery joondalup Say you have tables about products and orders, but now you want to allow one order to contain possibly many products (which the structure above did not allow). Data recovery kit You could use this table structure: CREATE TABLE products (
We know that the foreign keys disallow creation of orders that do not relate to any products. Data recovery kansas city But what if a product is removed after an order is created that references it? SQL allows you to handle that as well. Data recovery kali linux Intuitively, we have a few options:
To illustrate this, let’s implement the following policy on the many-to-many relationship example above: when someone wants to remove a product that is still referenced by an order (via order_items), we disallow it. Data recovery knoxville If someone removes an order, the order items are removed as well: CREATE TABLE products (
Restricting and cascading deletes are the two most common options. Data recovery key RESTRICT prevents deletion of a referenced row. Data recovery kickass NO ACTION means that if any referencing rows still exist when the constraint is checked, an error is raised; this is the default behavior if you do not specify anything. Data recovery linux (The essential difference between these two choices is that NO ACTION allows the check to be deferred until later in the transaction, whereas RESTRICT does not.) CASCADE specifies that when a referenced row is deleted, row(s) referencing it should be automatically deleted as well. Data recovery lab There are two other options: SET NULL and SET DEFAULT. Data recovery los angeles These cause the referencing column(s) in the referencing row(s) to be set to nulls or their default values, respectively, when the referenced row is deleted. Data recovery live cd Note that these do not excuse you from observing any constraints. Data recovery laptop For example, if an action specifies SET DEFAULT but the default value would not satisfy the foreign key constraint, the operation will fail.
Analogous to ON DELETE there is also ON UPDATE which is invoked when a referenced column is changed (updated). Data recovery linux distro The possible actions are the same. Data recovery las vegas In this case, CASCADE means that the updated values of the referenced column(s) should be copied into the referencing row(s).
Normally, a referencing row need not satisfy the foreign key constraint if any of its referencing columns are null. Data recovery linux live cd If MATCH FULL is added to the foreign key declaration, a referencing row escapes satisfying the constraint only if all its referencing columns are null (so a mix of null and non-null values is guaranteed to fail a MATCH FULL constraint). Data recovery lifehacker If you don’t want referencing rows to be able to avoid satisfying the foreign key constraint, declare the referencing column(s) as NOT NULL.
A foreign key must reference columns that either are a primary key or form a unique constraint. Data recovery miami fl This means that the referenced columns always have an index (the one underlying the primary key or unique constraint); so checks on whether a referencing row has a match will be efficient. Data recovery macbook Since a DELETE of a row from the referenced table or an UPDATE of a referenced column will require a scan of the referencing table for rows matching the old value, it is often a good idea to index the referencing columns too. Data recovery mac free Because this is not always needed, and there are many choices available on how to index, declaration of a foreign key constraint does not automatically create an index on the referencing columns.
More information about updating and deleting data is in Chapter 6. Data recovery micro sd Also see the description of foreign key constraint syntax in the reference documentation for CREATE TABLE.
Exclusion constraints ensure that if any two rows are compared on the specified columns or expressions using the specified operators, at least one of these operator comparisons will return false or null. Data recovery mac hard drive The syntax is: CREATE TABLE circles (