The previous chapter discussed how to create tables and other structures to hold your data. Now it is time to fill the tables with data. This chapter covers how to insert, update, and delete table data. We also introduce ways to effect automatic data changes when certain events occur: triggers and rewrite rules. The chapter after this will finally explain how to extract your long-lost data back out of the database.
When a table is created, it contains no data. The first thing to do before a database can be of much use is to insert data. Data is conceptually inserted one row at a time. Of course you can also insert more than one row, but there is no way to insert less than one row at a time. Even if you know only some column values, a complete row must be created.
CREATE TABLE products ( product_no integer, name text, price numeric );
An example command to insert a row would be:
INSERT INTO products VALUES (1, 'Cheese', 9.99);
The data values are listed in the order in which the columns appear in the table, separated by commas. Usually, the data values will be literals (constants), but scalar expressions are also allowed.
The above syntax has the drawback that you need to know the order of the columns in the table. To avoid that you can also list the columns explicitly. For example, both of the following commands have the same effect as the one above:
INSERT INTO products (product_no, name, price) VALUES (1, 'Cheese', 9.99); INSERT INTO products (name, price, product_no) VALUES ('Cheese', 9.99, 1);
Many users consider it good practice to always list the column names.
If you don't have values for all the columns, you can omit some of them. In that case, the columns will be filled with their default values. For example,
INSERT INTO products (product_no, name) VALUES (1, 'Cheese'); INSERT INTO products VALUES (1, 'Cheese');
The second form is a PostgreSQL extension. It fills the columns from the left with as many values as are given, and the rest will be defaulted.
For clarity, you can also request default values explicitly, for individual columns or for the entire row:
INSERT INTO products (product_no, name, price) VALUES (1, 'Cheese', DEFAULT); INSERT INTO products DEFAULT VALUES;
To insert a hole table into another, you can also use INSERT. Both tables have to contain the same columns with the same name and datatype.
INSERT INTO products (SELECT product_no, name, price FROM products_old);
If the columns in both tables are in the same order you can also replace the column names by an asterisk:
INSERT INTO products (SELECT * FROM products_new);
This is very useful when you want to remerge a temporary modified table with the genuine table which was the base of the temporary one. Simply truncate the genuine table with TRUNCATE TABLE products; and than use the above INSERT statement.