Discuss format conversion methods in MySQL

MySQL is one of the most popular relational database management systems. It supports many data types, including numbers, dates, strings, etc. However, in practical applications, we often need to convert one data type to another data type, such as converting a string to a number or date type, or converting a number type to a string type, etc. In MySQL, format conversion is a very important feature. In this article, we will discuss the format conversion methods in MySQL.

1. Introduction to data types

In MySQL, we mainly use the following data types:

1. Number type

In MySQL , digital types include integers and floating-point types. Commonly used integer types include TINYINT, SMALLINT, MEDIUMINT, INT and BIGINT, etc.; commonly used floating-point types include FLOAT and DOUBLE.

2. Date and time types

In MySQL, common date and time types include DATE, TIME, DATETIME and TIMESTAMP, etc.

3. String type

In MySQL, string types include CHAR, VARCHAR, TEXT, and BLOB. Among them, CHAR and VARCHAR both represent character types. , while TEXT and BLOB represent text types.

2. Data type conversion

In MySQL, data type conversion can be achieved through different functions. Below we will introduce the usage of these functions one by one.

1. CAST function

The CAST function is used to convert one data type to another data type, such as converting a string type to a numeric type. The syntax format is as follows:

CAST(expr AS type)

Among them, expr represents the expression to be converted, and type represents the converted data type, specifically including:

• BINARY[(N)]
• CHAR[(N)]
• DATE
• DATETIME
• DECIMAL[(M[,D])]
• SIGNED [INTEGER]
• TIME
• UNSIGNED [INTEGER]

For example, we can use the CAST function to convert the string type "123" to the numeric type INT, the specific code As follows:

SELECT CAST("123" AS INT);

2. CONVERT function

The CONVERT function is similar to the CAST function and can also be used for data type conversion. Its syntax format is as follows:

CONVERT(expr,type)

Among them, expr represents the expression to be converted, and type represents the target data type. In MySQL, the CONVERT function supports the same types as the CAST function.

For example, we can use the CONVERT function to convert the value returned by the NOW function of the date and time type into a string type. The specific code is as follows:

SELECT CONVERT(NOW(),CHAR);

This will return a value in the format of "YYYY-MM -DD ​​HH:MM:SS" string.

3. Precautions for data type conversion

When using data type conversion in MySQL, you need to pay attention to the following points:

1. Type conversion sequence

In MySQL, data type conversion is not arbitrary and needs to be carried out in a certain conversion order. The specific rules are as follows:

• If an operand is NULL, then conversion The result is also NULL.
• If either operand is DECIMAL, both operands are converted to DECIMAL.
• If either operand is DOUBLE, both operands are converted to DOUBLE.
• If either operand is FLOAT, both operands are converted to FLOAT.
• If all operands are integers, the result is integers.
• Otherwise, all operands are converted to string form, and the result is also of string type.

For example, for the following expression:

SELECT 1 + "2";

MySQL will first convert "2" of string type into 2 of numeric type, and then perform the addition operation. The final result is 3.

2. Implicit conversion and explicit conversion

In MySQL, there are also implicit conversion and explicit conversion.

Implicit conversion means that MySQL automatically converts a certain data type to another data type to adapt to the operation requirements of the expression. For example, when operating on a string type and a numeric type, MySQL will automatically convert the string type to a numeric type before performing the operation.

Explicit conversion is to clearly specify the type conversion required in the SQL statement, such as using the CAST or CONVERT function.

3. The problem of data precision loss

When performing type conversion, you need to pay attention to the problem of data precision. For example, when converting floating-point data to integer data, a loss of precision occurs, which may affect the accuracy of calculation results. Therefore, when performing data type conversion, you need to pay attention to details to avoid problems with loss of data accuracy or calculation errors.

4. Summary

Data type conversion in MySQL is an essential part of database development. Through the introduction of this article, we have learned about the format conversion method in MySQL and the details that need to be paid attention to. question. In the actual development process, we should choose the appropriate conversion method according to the specific situation to avoid problems such as data type mismatch or calculation errors, and further improve the performance and stability of MySQL applications.

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