How do I create an index in SQL? SQL statements that create indexes

The SQL statement to create an index is Create Index IndexName Ontable Name(Column Name).

Create index indexname on table name (column name) is the most basic syntax for creating an index, and it has no limitations. If it is char or varchar, the length can be less than the actual length of the field. If it is a blob and text type, you must specify length. The next step is to modify the table structure (add indexes), which you specify when you create the table.

Disadvantages of indexing:

Indexes should be created with care. Because every time you add, update, or delete a row in a table, you have to modify all the indexes on that table. As a result, the more indexes you have, the more work the server needs to perform, which ultimately leads to lower performance.

Here are some basic guidelines you can follow when creating an index:

1. Create indexes for columns that you frequently use to retrieve data.

2. Don't create indexes for columns that have never been used as search keys.

3. Index columns used to join to improve join performance.

4. Avoid columns that contain too many null values.

5. Again, small tables don't need indexes, because in the case of small tables, it's usually faster for the server to scan the table than to look at the indexes first.

Advantages: 1. Greatly speed up the retrieval of data;

2. Create a unique index to ensure the uniqueness of each row of data in the database table;

3. Accelerate the connection between tables and tables;

4. When using grouping and sorting clauses for data retrieval, the time of grouping and sorting in the query can be significantly reduced.

Disadvantages: 1. The index needs to occupy physical space.

2. When adding, deleting, and modifying the data in the table, the index should also be dynamically maintained, which reduces the maintenance speed of the data.

History: Indexes first appeared in the West, mainly for medieval European religious works. After the 18th century, there began to be thematic indexes in the West, and by the end of the 19th century, content analysis indexes were widely used.

Indexes in China appeared relatively late. It is generally believed that the "Rhyme of the Names of the Two Han Books" compiled by Fu Shan at the end of the Ming Dynasty is the earliest surviving index of personal names. During the Qianjia period of the Qing Dynasty, Zhang Xuecheng advocated the compilation of a comprehensive index of group books.

In the 20s of the 20th century, with the introduction of Western indexing theory and compilation technology, indexing and research in China in the modern sense flourished. In 1930, Qian Yaxin published "Index and Index Law", and in 1932, Hong Ye published "Introduction to Theory", which marked the rapid development of modern indexing theory and technology with Chinese characteristics.

In the 50s of the 20th century, computer technology was used for indexing. Since then, a large number of machine-compiled indexes have made profound changes in indexing theory, technology, and the form of indexing carriers.

Indexes are not covered in the SQL standard, but commercial relational database management systems generally support indexing mechanisms, but different relational database management systems support different types of indexes.

Indexes have become a very important part of relational databases. They are used as table pointers that contain the data they care about. With an index, you can find a specific record directly from a table, rather than having to scan the table sequentially and look it up one at a time.

For large tables, indexes are necessary. Without indexing, you have to wait hours, days, not seconds to get a result.

There are two types of indexes in a database: normal indexes and unique indexes. Let's take a look at each of these indexes.

The first is a normal index, create index indexname on mytable(username(length)); Specify the table when you create it. Syntax for deleting an index: drop index [indexname] on mytable;

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The second is a unique index, which is similar to a normal index, but there are differences, the difference is that the value of the index column must be unique, but null values are allowed. In the case of a combined index, the combination of column values must be unique.

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The preceding three figures are the data that is directly specified when you create an index, modify the table structure, and create a table. There are also four ways to add indexes to data tables, which are described below.

First way: alter table tbl name add primary key (column list): This statement adds a primary key, which means that the index value must be unique and cannot be null.

Second method: alter table tbl name add unique index name (column list): The value of the index created by this statement must be unique (except for null, which may appear more than once).

Method 3: alter table tbl name add index index name (column list): Add a normal index, the index value can appear multiple times.

Fourth method: alter table tbl name add fulltext index name (column list): This statement specifies that the index is fulltext and is used for full-text indexing.

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end to conclude.

Create an index. Modify the table structure.

Specify the data directly when you create a table.