MySQL與PHP的基礎(chǔ)與應(yīng)用專題之?dāng)?shù)據(jù)查詢語句

更新時間：2022年02月21日 16:50:33 作者：我是小白呀

MySQL是一個關(guān)系型數(shù)據(jù)庫管理系統(tǒng)，由瑞典MySQL AB 公司開發(fā)，屬于 Oracle 旗下產(chǎn)品。MySQL 是最流行的關(guān)系型數(shù)據(jù)庫管理系統(tǒng)之一，本系列將帶你掌握php與mysql的基礎(chǔ)應(yīng)用，本篇從數(shù)據(jù)查詢語句開始

概述

從今天開始, 小白我將帶領(lǐng)大家一起來補充一下數(shù)據(jù)庫的知識.

ORDER BY

在 SQL 中, 我們可以使用 ORDER BY 對查詢結(jié)果進(jìn)行一列或多列排序.

SQL 語句:

SELECT 列名1, 列名2 FROM 表名1, 表名2
ORDER BY 列名, 列名[ASC|DESC]

注意:

ASC 表示按升序排列, DESC 表示按降序排列
默認(rèn)情況下對列按升序排序

表記錄:

例 1

工資從低到高排序:

<?php

# 創(chuàng)建連接
$conn = mysqli_connect("localhost", "root", "admin", "study");

# 查看是否連接成功
if ($conn) {
    echo "服務(wù)器連接成功!\n";
} else {
    echo mysqli_connect_error();
}

# SQL語句, 條件查詢
$SQL = "SELECT * FROM user 
        ORDER BY salary";

# 執(zhí)行SQL語句
$result = mysqli_query($conn, $SQL);

# 查看是否執(zhí)行成功
if ($result) {
    echo "SQL語句執(zhí)行成功!\n";
} else {
    echo mysqli_error($conn);
}

# 調(diào)試輸出
while ($line =  mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 關(guān)閉連接
mysqli_close($conn);

?>

輸出結(jié)果:

服務(wù)器連接成功!
SQL語句執(zhí)行成功!
Array
(
[id] => 3
[username] => 王富貴
[password] => 123456
[sex] => 1
[salary] => 1000.00
)
Array
(
[id] => 4
[username] => 李大爺
[password] => 123123
[sex] => 1
[salary] => 1500.00
)
Array
(
[id] => 2
[username] => 張美麗
[password] => 123456
[sex] => 2
[salary] => 5000.00
)
Array
(
[id] => 5
[username] => 王二狗
[password] => 123123
[sex] => 1
[salary] => 6000.00
)
Array
(
[id] => 1
[username] => 我是小白呀
[password] => 123456
[sex] => 2
[salary] => 30000.00
)

例 2

工資從高到低排序:

<?php

# 創(chuàng)建連接
$conn = mysqli_connect("localhost", "root", "admin", "study");

# 查看是否連接成功
if ($conn) {
    echo "服務(wù)器連接成功!\n";
} else {
    echo mysqli_connect_error();
}

# SQL語句, 條件查詢
$SQL = "SELECT * FROM user 
        ORDER BY salary DESC";

# 執(zhí)行SQL語句
$result = mysqli_query($conn, $SQL);

# 查看是否執(zhí)行成功
if ($result) {
    echo "SQL語句執(zhí)行成功!\n";
} else {
    echo mysqli_error($conn);
}

# 調(diào)試輸出
while ($line =  mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 關(guān)閉連接
mysqli_close($conn);

?>

LIMIT

我們可以在 SELECT 語句中使用 LIMIT 來約束要返回的記錄數(shù), 通常使用 LIMIT 實現(xiàn)分頁.

SQL 語句:

SELECT 列名1, 列名2 FROM 表1, 表2 LIMIT [偏移量] 行數(shù)

注意:

第一行的偏移量是 0, 而不是 1
行數(shù)為返回最大的行數(shù)

例 1

取前兩個數(shù)據(jù):

<?php

# 創(chuàng)建連接
$conn = mysqli_connect("localhost", "root", "admin", "study");

# 查看是否連接成功
if ($conn) {
    echo "服務(wù)器連接成功!\n";
} else {
    echo mysqli_connect_error();
}

# SQL語句
$SQL = "SELECT * FROM user 
        LIMIT 2";

# 執(zhí)行SQL語句
$result = mysqli_query($conn, $SQL);

# 查看是否執(zhí)行成功
if ($result) {
    echo "SQL語句執(zhí)行成功!\n";
} else {
    echo mysqli_error($conn);
}

# 調(diào)試輸出
while ($line =  mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 關(guān)閉連接
mysqli_close($conn);

?>

輸出結(jié)果:

服務(wù)器連接成功!
SQL語句執(zhí)行成功!
Array
(
[id] => 1
[username] => 我是小白呀
[password] => 123456
[sex] => 2
[salary] => 30000.00
)
Array
(
[id] => 2
[username] => 張美麗
[password] => 123456
[sex] => 2
[salary] => 5000.00
)

例 2

取第 2, 3 條數(shù)據(jù):

<?php

# 創(chuàng)建連接
$conn = mysqli_connect("localhost", "root", "admin", "study");

# 查看是否連接成功
if ($conn) {
    echo "服務(wù)器連接成功!\n";
} else {
    echo mysqli_connect_error();
}

# SQL語句
$SQL = "SELECT * FROM user 
        LIMIT 1, 2";

# 執(zhí)行SQL語句
$result = mysqli_query($conn, $SQL);

# 查看是否執(zhí)行成功
if ($result) {
    echo "SQL語句執(zhí)行成功!\n";
} else {
    echo mysqli_error($conn);
}

# 調(diào)試輸出
while ($line =  mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 關(guān)閉連接
mysqli_close($conn);

?>

輸出結(jié)果:

服務(wù)器連接成功!
SQL語句執(zhí)行成功!
Array
(
[id] => 2
[username] => 張美麗
[password] => 123456
[sex] => 2
[salary] => 5000.00
)
Array
(
[id] => 3
[username] => 王富貴
[password] => 123456
[sex] => 1
[salary] => 1000.00
)

GROUP BY

從字面上理解, GROUP BY 表示根據(jù)某種規(guī)則對數(shù)據(jù)進(jìn)行分組. 它必須配合聚合函數(shù)進(jìn)行使用, 對數(shù)據(jù)進(jìn)行分組后可以進(jìn)行 COUNT, SUM, AVG, MAX 和 MIN 等運算.

SQL 語句:

SELECT 列名, 聚合函數(shù)(列名)
FROM 表名
GROUP BY 列名

注意:

GROUP BY 可以對一列或多列進(jìn)行分組

表記錄:

例 1

<?php

# 創(chuàng)建連接
$conn = mysqli_connect("localhost", "root", "admin", "study");

# 查看是否連接成功
if ($conn) {
    echo "服務(wù)器連接成功!\n";
} else {
    echo mysqli_connect_error();
}

# SQL語句
$SQL = "SELECT gender, count(*) FROM student 
        GROUP BY gender";

# 執(zhí)行SQL語句
$result = mysqli_query($conn, $SQL);

# 查看是否執(zhí)行成功
if ($result) {
    echo "SQL語句執(zhí)行成功!\n";
} else {
    echo mysqli_error($conn);
}

# 調(diào)試輸出
while ($line =  mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 關(guān)閉連接
mysqli_close($conn);

?>

輸出結(jié)果:

服務(wù)器連接成功!
SQL語句執(zhí)行成功!
Array
(
[gender] => 1
[count(*)] => 3
)
Array
(
[gender] => 2
[count(*)] => 6
)

例 2

<?php

# 創(chuàng)建連接
$conn = mysqli_connect("localhost", "root", "admin", "study");

# 查看是否連接成功
if ($conn) {
    echo "服務(wù)器連接成功!\n";
} else {
    echo mysqli_connect_error();
}

# SQL語句
$SQL = "SELECT grade, count(*) FROM student 
        GROUP BY grade";

# 執(zhí)行SQL語句
$result = mysqli_query($conn, $SQL);

# 查看是否執(zhí)行成功
if ($result) {
    echo "SQL語句執(zhí)行成功!\n";
} else {
    echo mysqli_error($conn);
}

# 調(diào)試輸出
while ($line =  mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 關(guān)閉連接
mysqli_close($conn);

?>

輸出結(jié)果:

服務(wù)器連接成功!
SQL語句執(zhí)行成功!
Array
(
[grade] => 高三
[count(*)] => 2
)
Array
(
[grade] => 高一
[count(*)] => 4
)
Array
(
[grade] => 高二
[count(*)] => 3
)

HAVING

HAVING 可以解決 WHERE 關(guān)鍵詞無法與聚合函數(shù)一起使用的問題. HAVING 可以對分組后的各組數(shù)據(jù)進(jìn)行篩選.

SQL 語句:

SELECT 列名, 聚合函數(shù)(列名)
FROM 表名
GROUP BY 列名
HAVING 聚合函數(shù)(列名) 條件

例 1

<?php

# 創(chuàng)建連接
$conn = mysqli_connect("localhost", "root", "admin", "study");

# 查看是否連接成功
if ($conn) {
    echo "服務(wù)器連接成功!\n";
} else {
    echo mysqli_connect_error();
}

# SQL語句
$SQL = "SELECT grade, count(*) FROM student 
        GROUP BY grade
        HAVING count(*) > 2";

# 執(zhí)行SQL語句
$result = mysqli_query($conn, $SQL);

# 查看是否執(zhí)行成功
if ($result) {
    echo "SQL語句執(zhí)行成功!\n";
} else {
    echo mysqli_error($conn);
}

# 調(diào)試輸出
while ($line =  mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 關(guān)閉連接
mysqli_close($conn);

?>

輸出結(jié)果:

服務(wù)器連接成功!
SQL語句執(zhí)行成功!
Array
(
[grade] => 高一
[count(*)] => 4
)
Array
(
[grade] => 高二
[count(*)] => 3
)

例 2

<?php

# 創(chuàng)建連接
$conn = mysqli_connect("localhost", "root", "admin", "study");

# 查看是否連接成功
if ($conn) {
    echo "服務(wù)器連接成功!\n";
} else {
    echo mysqli_connect_error();
}

# SQL語句
$SQL = "SELECT gender, count(*) FROM student 
        GROUP BY gender
        HAVING count(*) > 3";

# 執(zhí)行SQL語句
$result = mysqli_query($conn, $SQL);

# 查看是否執(zhí)行成功
if ($result) {
    echo "SQL語句執(zhí)行成功!\n";
} else {
    echo mysqli_error($conn);
}

# 調(diào)試輸出
while ($line =  mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 關(guān)閉連接
mysqli_close($conn);

?>

輸出結(jié)果:

服務(wù)器連接成功!
SQL語句執(zhí)行成功!
Array
(
[gender] => 2
[count(*)] => 6
)

GROUP_CONCAT

當(dāng)我們使用GROUP BY可以獲取分組, 但是無法看到具體信息. 這時我們就可以使用GROUP_CONCAT來獲取具體信息. GROUP_CONCAT配合GROUP BY一起使用, 可以將一列的值按照指定分隔符進(jìn)行拼接 (默認(rèn)為 “,”).

SQL 語句:

GROUP_CONCAT(列名[排序列 ASC/DESC] [分隔符])

例 1

<?php

$conn = mysqli_connect("localhost", "root","admin","study");

if ($conn) {
    echo "數(shù)據(jù)庫鏈接成功";
} else {
    echo mysqli_connect_error();
}

# SQL語句
$SQL = "SELECT grade, count(*), GROUP_CONCAT(student_name)
        FROM student
        GROUP BY grade";

# 執(zhí)行
$result = mysqli_query($conn, $SQL);

# 查看是否執(zhí)行成功
if ($result) {
    echo "SQL 語句執(zhí)行成功!";
}else {
    echo mysqli_error($conn);
}

# 調(diào)試輸出
while ($line = mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 關(guān)閉連接
mysqli_close($conn);

?>

輸出結(jié)果:

數(shù)據(jù)庫鏈接成功SQL 語句執(zhí)行成功!Array
(
[grade] => 高一
[count(*)] => 4
[GROUP_CONCAT(student_name)] => 王富貴,楊美麗,楊美麗,楊美麗
)
Array
(
[grade] => 高三
[count(*)] => 2
[GROUP_CONCAT(student_name)] => 楊美麗,楊美麗
)
Array
(
[grade] => 高二
[count(*)] => 3
[GROUP_CONCAT(student_name)] => 楊美麗,楊美麗,趙有才
)

例 2

<?php

$conn = mysqli_connect("localhost", "root","admin","study");

if ($conn) {
    echo "數(shù)據(jù)庫鏈接成功";
} else {
    echo mysqli_connect_error();
}

# SQL語句
$SQL = "SELECT gender, count(*), GROUP_CONCAT(student_name)
        FROM student
        GROUP BY gender";

# 執(zhí)行
$result = mysqli_query($conn, $SQL);

# 查看是否執(zhí)行成功
if ($result) {
    echo "SQL 語句執(zhí)行成功!";
}else {
    echo mysqli_error($conn);
}

# 調(diào)試輸出
while ($line = mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 關(guān)閉連接
mysqli_close($conn);

?>

輸出結(jié)果:

數(shù)據(jù)庫鏈接成功SQL 語句執(zhí)行成功!Array
(
[gender] => 1
[count(*)] => 3
[GROUP_CONCAT(student_name)] => 楊美麗,楊美麗,楊美麗
)
Array
(
[gender] => 2
[count(*)] => 6
[GROUP_CONCAT(student_name)] => 王富貴,楊美麗,楊美麗,楊美麗,楊美麗,趙有才
)

DISTINCT

DISTINCT用于在查詢中返回父唯一不同值, 支持單列或多列. 在實際的應(yīng)用中, 表中的某一列含有重復(fù)值是很常見的. 如果在查詢數(shù)據(jù)時, 希望得到某列的所有不同值, 可以使用 DISTINCT.

SQL 語句:

SELECT DISTINCT 列名1, 列名2 FROM 表名

例 1

<?php

$conn = mysqli_connect("localhost", "root","admin","study");

if ($conn) {
    echo "數(shù)據(jù)庫鏈接成功";
} else {
    echo mysqli_connect_error();
}

# SQL語句
$SQL = "SELECT DISTINCT grade FROM student";

# 執(zhí)行
$result = mysqli_query($conn, $SQL);

# 查看是否執(zhí)行成功
if ($result) {
    echo "SQL 語句執(zhí)行成功!";
}else {
    echo mysqli_error($conn);
}

# 調(diào)試輸出
while ($line = mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 關(guān)閉連接
mysqli_close($conn);

?>

輸出結(jié)果:

數(shù)據(jù)庫鏈接成功SQL 語句執(zhí)行成功!Array
(
[grade] => 高三
)
Array
(
[grade] => 高一
)
Array
(
[grade] => 高二
)

例 2

<?php

$conn = mysqli_connect("localhost", "root","admin","study");

if ($conn) {
    echo "數(shù)據(jù)庫鏈接成功";
} else {
    echo mysqli_connect_error();
}

# SQL語句
$SQL = "SELECT DISTINCT gender FROM student";

# 執(zhí)行
$result = mysqli_query($conn, $SQL);

# 查看是否執(zhí)行成功
if ($result) {
    echo "SQL 語句執(zhí)行成功!";
}else {
    echo mysqli_error($conn);
}

# 調(diào)試輸出
while ($line = mysqli_fetch_assoc($result)) {
    print_r($line);
}

# 關(guān)閉連接
mysqli_close($conn);

?>

輸出結(jié)果:

數(shù)據(jù)庫鏈接成功SQL 語句執(zhí)行成功!Array
(
[gender] => 1
)
Array
(
[gender] => 2
)

到此這篇關(guān)于MySQL與PHP的基礎(chǔ)與應(yīng)用專題之?dāng)?shù)據(jù)查詢語句的文章就介紹到這了,更多相關(guān)MySQL 數(shù)據(jù)查詢語句內(nèi)容請搜索腳本之家以前的文章或繼續(xù)瀏覽下面的相關(guān)文章希望大家以后多多支持腳本之家！

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